What are you doing for the replacement gaskets for the 70 and some later 351C-4V carburetor base gaskets? These were D0AE-9447-B or D0AZ-9447-C, or FelPro 60100 or Mahle G32996. They have a metal sandwich layer blocking manifold heater openings. They’ve been discontinued everywhere. Perhaps cutting two gaskets and sandwiching a piece of steel sheet between them?
I am using two different gaskets. Probably not the correct ones. But they work.
60148
60059
Of course I am going to use the plastic pcv plenum for the boss 302. I could not find the correct one for a 351C. They have the ones for the 351W all over but they are 1 inch thick so don’t know if they will work.
A caution note on that combination if you are using them together (my initial read had them stacked, probably not what you’re doing but that shaped my initial answer that follows). I had the center X section of a 60100 break loose and foul a secondary throttle plate partially open. The car wanted to do about 40 mph and wouldn’t slow. Had to kill the ignition, coast and get towed. It was scary, at least I remembered to not take the ignition switch far enough to lock the steering. If you use the combination I’d be concerned that the center part was even less restrained.
The 60059 looks similar but without the metal inset. If it holds up it should work. I’m going to get one and check it to the bottom of a Holley 4160. Maybe these could be used to sandwich a metal piece.