1975 Mercury Headers

 I see Hooker quit building the 6125 HKR too . 

So….. I sure can’t justify a performance 400 build in this chassis with cast iron exhaust manifolds. 

Maybe I will put an LS motor in it , that seems to be about the only platform being supported by the aftermarket . 

Ah, that's a reasonable and IMO feasible goal. The 351M is the same as a 400 besides the crank and rods. You can easily rebuild the engine with a 400 crank and rods, raise the compression enough to gain efficiency. Vince can tell you how he built a nice 400 that will definitely move far better than the stock stuff.

It would be best to acquire closed chamber heads like the Australian 2V versions, then it can be less likely to ping or need too much retarding of the timing. The right cam(a custom cam will) does allow for regular gas if you keep the compression under about 9.7:1, open chamber heads likely would have to have notably less compression in all cases.

That kind of build with stock crank and rods, block, OEM heads, should make 400lbsft of torque at  streetable rpm levels. The intake and carb etc, it matters, the exhaust manifolds, headers would be great but the stock manifolds wouldn't kill it completely.

Great, and good find for the low mileage 400. I think I'd stick with the cast iron heads and limit the rpm's since it's a 400 and the headers are not to be found. I've priced both ways for a long while, and I'm going to do the 4V heads on my complete 351 truck, while choosing the TFS for my other one, also 351C with similar headers and cam, intake etc. The aluminum heads will end up being way over $2k no matter what, while the cast iron might be in the $1200 or so range.

I know that nobody is close to me, but maybe one day someone will be able to buy my 400 block and crank, plus finished rods and pistons(polished and balanced with TRW .030 over). I began a 400 project in the late 80's, and the machine shop finally went under when the father died. They couldn't find my crank, so I later bought another 400 just for the crank which is still stock. This block was finished basically, .030 over with screw in freeze plugs, plus 4-bolt main caps. It's been stored in my garage since I got it back, rods/pistons inside in a box. I won't be building one for sure, but somebody could appreciate them.

As I understand it that's easy to get. That 400 would love a set of 4V heads and make better use of them. The do make spacers to adapt 351C intakes to those.

If my 351CJ gets hurt I might do a "400CJ" build. Just because.