If you go with hydraulic lifters and even if solid be sure and check the clearances in the lifter bores. Many never look at this. With a wet lifter galley you can loose a lot of oil around the lifters if the clearance is excessive. Some bore them out and put bronze sleeves in. If you can find a mfg. of lifters that will work with you and grind the to an oversize dia. that will allow you to hone the bores out to fit the new lifter size. In this day and time difficult to find someone like that.
I worked in one of the old NASCAR shops in Asheville NC in the 60's & 70's and the lifter bores were one area my boss always stressed be right. Been too long I cannot give you correct clearance but would probably look at the Ford spec for the Boss 351 for suggestion.
The side oil 427 eliminated that issue and that is why Ford used it for racing applications. You need the oil in the crank bearings not into lifters with very little side load that need minimum oil supply.
When we were building an all out race engine we did not use head gaskets either. The heads were lapped to the block and no gasket used. Sprayed silver Rustolieum paint on and bolted them together. I have seen Pontiac 303 Trans Am engine go over 2,000 miles on the tract in Buck Bakers car and never gave an issue. The reason we did that was to get maximum compression which was usually up around 13 or 14 to one. Yes way to high unless you are using racing fuel so keep that in mind. I understand that when Bill Elliott was running 351 his was in the 15 to 16 to one range. I would suspect he used no gaskets either. The engine with that compression makes a totally different sound, very very crisp.
You might also consider doing some Cryogenic treatment to the engine components. Might talk with them about the C-6 components also. It does work and pretty much everything in a NASCAR or INDY car that is mechanical goes to Cryo treatment. Makes brake rotors last much longer in race usage. It totally redefines the grain structure of the metal. Works on guns also, lol.
I am sure you are going to convert the core plugs to screw in also or at least pin them with drive screws to prevent blowing them out.
It is interesting that you are using the C-6. I am currently working on a 72 Q code CJ Vert that came with C-6. I wanted to do some track days with the car and I do have a 4 speed set up I could use but wanted to keep car original since only 330 were built.
It currently has a 31 spline Detroit locker 9" that will have to go for any road race capability for sure. Might work for drifting or drag racing but not road racing.
I was also adding a 6 point roll cage and harness so that they do not limit my speed. I had hoped to have it ready for April 2019 for the 55 year anniversary celebration of the Mustang in Charlotte. They will have track day and run the Roval. NASCAR is actually going to run the Roval in an upcoming race I think in Sept. They run the road course in the infield then exit onto the oval at turn one then two and back stretch 3 & 4 and back to road course. It is a blast has lots of elevation changes on the road course. I was limited to 90 mph in a vert with no harness or cage last year. I might not get finished it looks like the MOM, Mustang Owner's Museum, is going to want my original 73 Mach 1 for display. I find out this month. If so I have to drop work on the 72 Q to get the Mach 1 cleaned up and ready for the grand opening in April 2019.
They are doing a pony run from Washington state down through California and across the U.S. to Charlotte for the celebration. You should make the trip. There were a couple that came from California last year.
Do keep us informed on what you end up with since I am doing about the same but with a vert..
