oiling mods for dummies

I would take a close look at the 308 heads - the shaft oil feed holes specifically and see if they are blocked off in the heads (set screw or plugging the passage at the head gasket end). The bars look fine to me.
If I had to guess you have issues with clearances, but you also have two competing oiling systems for the top end (shafts through the unblocked RPM feeds, plus pushrods from the lifters) which basically means the lower end never got as much oil as it should have.
The valves are cracked from loss of valve control - not heat or coil bind. You might be close to coil bind because we can't tell by the picture but that's not going to crack heads. Slamming onto the seat would - aka not enough valve spring for the cam. Hence my question on the cam. Basically - you got a mess.
Oh - and you can't "take up clearance" with rings. You can run the wrong piston and the right ring but you caused some big issues with wear, noise, and ring seal - you didn;t take up anything.
Sorry dude- that "builder" fucked you over huge. My best advice - get it to someone who can go through it top to bottom. Oh - on the cover pickup - have one made. Instructions are available if you look hard enough. Mopar engine & track manuals from the 70s should have them because the placement of the hole in the cover is important. Easiest way is to weld a fitting onto the cover, then fab up a pickup