Sorry, never heard of a short or long oil filter boss.
I have no idea what your talking about for the oil pump Eccentric. No Mopar oil pumps are driven off and eccentric. None at all.
Hydraulic lifters are shorter than hydraulic roller or solid roller lifters. Most companies supply AMC lifters that will oil through the pushrod if need be. Then you just need longer pushrods that oil through. The magnum head will work with whatever style camshaft you choose. It’s just a lifters that must be able to oil through the pushrods.
I don’t know how you come up with the comment, are these magnum blocks better off as boat anchors. That’s one of the most insane things I’ve ever read. Since your block is an early one, check to see for sure if the death of the block hasn’t boiling passage to oil through the hedge. It should be there. IF! The oiling hole is in the block to oil the heads, I would certainly use LA heads.
I am not sure of what exhaust manifolds will work with your car.
