Ping at cruise speed

I was going to suggest to use a piston stop to check TDC is correct on your balancer, but based on your latest post maybe it is not ignition ping?  When did the ping start?  Was it around the same time as the vibration?  Do you get the noise under load only or will it make it when revving in neutral?

The speedometer cable can make a noise, but it usually sounds like it's coming from inside the car, not the engine.  I would also agree that exhaust leaks generally get louder with more throttle, and sometimes go away with some heat in the engine.  If you have exhaust manifolds, they do crack, in particular the passenger side.  I don't think the convertor would be the issue, unless it was bad/out of balance and reorienting it likely wouldn't fix that.  You could unbolt the converter and try firing up the engine to see if it still has the vibration.

Can you do a video so we can hear the sound too? 

On the intake, if you do have port mismatch, I am not sure how great of a seal you will get with the gaskets.  That could be part of the vacuum leak issue.  15 inches seems a little lower than what I would expect for a stock 351C 2-V engine at low RPM. 

Inkara1 wrote: ... I verified TDC by rotating it until the pointer was at 0, then stuck a camera in the No.1 cylinder spark plug hole. It's either right on or extremely close. ...

The TDC is the one for #1 cylinder on its compression stroke. Put your finger over the hole as it approaches TDC, if it doesn't blow your finger out then it's on the exhaust stroke(the exhaust valve is open at that time).

So I'm happy I don't need to go through the work of changing the bearings, but sad that I'm no closer to finding the issue.

On a C6 transmission, can I get the converter pushed back far enough to be able to run the engine without it hooked up without having to unbolt the bellhousing from the engine?

When I changed the valve seals, I torqued the rocker arms to the head to shop manual spec (non-adjustable). I did so after I did each cylinder, so the torquing was always done with the valves not being opened. I have a leaking rubber valve cover gasket, so I need to swap the gaskets anyway, so I guess I could retorque the rocker arms while the covers are off. Anything special to torquing them? Is it best to torque with the valves at rest or with them opened?

For inspecting the flexplate, do I just remove the converter nuts, push it back as far as I can, and then shove the inspection camera up there? (It looks like a stock converter, with studs and nuts.)

and i'll throw this out there, my winter daily salt eater has what i believe to be an extremely broken flex plate. it makes a of a racket that i suppose could be on a lesser scale mistaken for ignition ping? as it is it sounds as if the engine identifies as a diesel and who am i to question that? it sounds like it's ready to kick out a rod but shows good oil pressure and goes down the road 80mph playing it's beat. fine with me, keep rollin baby, eat that salt! keeps my summer daily from having to chew on it

if the flex is cracked up near around the crank bolt circle you might still get a glean on it through the inspection cover with a flashlight?

retorquing the nonadjustable rocker arms won't change anything but if you're game have a look at how much preload is on your lifter plungers. as you described go through the firing order each cylinder at a time with the lifters on the cam base circle. i really need to find out for myself where that is but until then i guess TDC on the compression stroke is generally accepted? so loosen the 2 rockers on the cylinder that is in TDC firing position (chalk the balancer for each 90*ish) and now it's time to get a feel for what is called Zero Lash. this is when the rocker is just snug enough that any more tightening of the bolt will begin to depress or preload the lifter plunger, send a couple minutes getting the feel, just the bare socket or even no socket just your fingers turning the rocker bolt and your other hand your fingers slightly rotating the pushrod back & forth. snug the bolt and you'll feel a miniscule drag on the pushrod, that point is zero lash. paint pen or chalk mark the bolt on 1 side your choice top bottom whatever so you can count how much revolution the bolt turns from zero lash to bottom tight. maybe not necessary to go to TQ spec but snug hard enough that the bolt has effectively stopped turning. the distance from Zero Lash to bottom tight is your lifter plunger preload. bolt rotation can be converted to decimal fractions of an inch by using the thread pitch of the bolt and a little math. IIRC the bolt is 5/16-18 so 18 threads (revolutions) per inch. 1 / 18 =  .055" per 1 full revolution of the bolt. if the bolt turned 1/2 a turn from Zero Lash to bottom tight your LPP is .0275" give or take because of the rocker ratio and i've never sat down and figured out how that affects the deal. document each rocker and you can shim the pedestal to equalize your preload. shim kits are available or you can use almost any metal to make your own, some guys even use brass and that gets criticized but i guess it works. sandwiched between the pedestal and the head it doesn't take abuse

https://www.summitracing.com/parts/fms-m-6529-a302/make/ford

so the end head bolt bosses are round at the top? IDK what the Aus heads did for the 2 & the dot

yes all 1972 engines came with the same flat top pistons. today's calculation a 72 2V should make 8.6:1 static squeeze, it would take a Very small cam to make trouble from there

The new converter has bolts instead of nuts, so I could check if the vibration goes away with it disconnected. But I'm wondering now if this is caused by my crankshaft that has outrageous end play because the thrust bearing wore a groove into the crank as deep as the bearing itself. Either that or check the valvetrain to make sure everything is torqued to spec.

crankshaft free end play is called out at .004" - .010" in the factory shop manual

it's amazing what will run fairly well but when you get your peepers on it you're like How The did that work?

from what you say about the excessive crank free play nothing good can happen from this point if you keep running it

My main worry is that the Fairlane Club shows I go to can be pretty far away. It's a six-hour drive to the new Nor-Cal meet in May, and then the one I'm hosting in October in the LA area is 2.5-3 hours away. I just want to be reasonably sure the bottom end won't explode on the freeway hours away from home.

I am wondering I the previous owners swapping of transmissions buggered up the crank. I know you said the c4 flex has clearance, but maybe there was a combo in there that put too much pressure on the crank. It sounds like who ever owned your car before really cobbled things together, and now you are cleaning up his mess.

I think the engine needs to be pulled and a deeper dive inside it needs to happen. Maybe a used 351 is a good alternative. Or even a 400 or 460 if you can find a donor car. Any midzied 70s full frame Ford or Tbrid and Lincoln Mark IV and V can supply what you need for a swap.