1968 390 gt oil leaks

Nice car, I'm sure you will drive that a bunch and keep it going. The rear main seal is likely to have a minor leak, typically you live with that due to the R&R labor.

I'd hunt for the best valve cover gaskets you can buy. Find real steel core gaskets, and skip any cork gaskets. There will be some choices, any quality version should do.

The intake itself may have a blown out rear gasket, the back piece filling the gap to the block. Reach back there along the back edge of the intake, and see if you can feel the gasket or RTV if it was filled with that, and how much oil is there along that section. If the intake back rail is leaking badly, it is possible to fix that without removing the intake. I've done it on my old 85 Crown Vic 351W, it took a lot of cleaning and a thick smearing of Ultra Black RTV. That section is just a wall to keep the oil inside the engine, no forces except heat. Cleaned well, very well, RTV will stick and last forever there.

You can use any oil you'd like, but the engine needs the old Zinc additive stuff, for the flat tappet cam and lifters. A synthetic oil is never bad, but they do have more cleaning additives in them, so they will remove buildups that might have been keeping oil from escaping past worn out seals. So expect to find some seals leaking, the front and rear of course, all but the rear should be manageable in any case.

I see there are lots of choices, and most mention cork, on Summit. I'd go with that 7568 part you mentioned. It looks to be a good material with some kind of internal more rigid core.

I'm sorry they don't have a steel type like what was stock on Fox Mustang 302's. I've been spoiled by those, they can last forever. What you found is the good kind in the middle quality gasket, way better than cork for sure.

Fixing leaks is often about cleaning many times as you trace the sources, and one at a time fixing them. Often some bolts are a little loose, and that can help to snug everything up.

Cork gaskets leak in much shorter time than anything else, cork should never be used as a gasket material IMO. I'd never use cork even in an emergency, Ultra Black RTV is vastly better under any conditions than cork. I'd gladly make a gasket with RTV instead of using cork, and I have.

I've watched Subaru mechanics install dozens of valve cover gaskets, their OEM cork gaskets. They coated the VC surface completely with RTV, and after a short time, the rest of the gasket before bolting them to the head. The RTV sealed up the entire surface, and the cork was the core material. Over enough time, the cork shrinks and/or breaks down.

Without some other bonding/sealing material, cork always leaks. So why ever choose cork, it's terrible gasket material. It survives a factory warrantee period, usually, but that's not good enough.

Another issue I can into today was brake fluid leaking from a new master cylinder that was installed. I can see the line coming into the bottom port the the master cylinder is wet, and there looks like spray behind the drivers-side shock tower. I'll try and get a few pics and post them. 

The master cylinder was replaced as the brakes were "grabby" my dad said. The front calipers were also replaced. 

If it is a short line, that you could bend to shape with raw Nicopp, with the two fittings on it, take it to someone who can form the flared ends for you, and the bends of the line will allow it to fit easily. Long lines are a different matter, those usually have to be made(form the ends) in the vehicle.

I helped a friend about ten years ago with a MC swap, his 90 Lincoln LSC had a bad MC, and those are massive and complicated(TEVES brake system). One of his old lines(left front) would not come out of the MC, rounded the fitting off finally. I bought some Nicopp line locally, about three feet, and had a line bent up in about ten minutes. The ends took a good five minutes each, so the whole line was about an hour with driving to a local hydraulics business that carried the Nicopp line.

The master cylinder has seals that are simple to replace. Pull it out and return it for a new one. Once you have gone through all the brake components, you will be confident that it will stop like it is supposed to.

so much for the good mechanic. replacing a master on a 1968 anything should've included the lines to the prop valve, just because.

you can see that the line is tweaked from the 1st try at loosening it, the 1st thing i'd do is loosen the tube nut and try to correct the angle of the line coming out of the master. it should be coming out straight down rather than the 10* angle it is now. hopefully the flare in the old line will reseal once it's put back straight.

next worst case you buy a short brake line with ends formed and nuts installed and bend it yourself, it's easy

next worst case you get yourself a complete brake line kit and install it yourself, or by a slightly better mechanic. this is one source, there are others
https://www.inlinetube.com/pages/product-list?year=4212202341531695823&make=1828299879717471912&model=4907562634040960157#?f=6883668340566495367|

if you don't have them, buy a couple tubing wrenches that cover the sizes of tubing nuts on your car or just grab a set. if you're going to replace a line straight away, just cut the tube coming out of the tube nut and use a 6 point socket rather than dick with the wrenches. a good pair of diagonal cutters will do the trick and you can right on the tube nut solidly with the socket

as far as the valve cover gaskets go, if the engine runs good and you don't intend to pull the covers off for rocker lash adjustment between rounds, i'd skip the gaskets and use just RTV silicone. get the surfaces very clean and oil free and the covers will stay on with no bolts, but use the bolts to locate the covers until the RTV sets LOL. but seriously, RTV won't leak, ever, if you prep the surfaces well

relax, you got this man!

I have read about fluid migrating into the booster, however I feel the mechanic should have noticed this. I'm really thinking about replacing it all at this point (booster, MC, lines, etc). 

i realized something when i wasn't even trying, i think the reason the leaking line is all bent out of shape is because you have the wrong master on the car. if i'm right your original master had the line coming out of the unused boss on the side like the rear line, not the down schnozzle. i read somewhere why the masters are different but that brain cell took a leave, it may have been hardtop / convertible or something to do with the brake config whether they're manual drum, power drum, manual disc/drum or power disc/drum i forget but there's a reason the master was different from the factory. different applications

what is your brake configuration?

or it could just be bent