part of the compression loss in 1974 is due to the deep piston dishes, so you're not going to get all the way to 10:1 CR.
i know Yardley did an old head onto 455 conversion back in the day, but i think he used the 430 rockers. which caused issues, because the 430 and 455 use different diameter pushrods.
yes, you should be able to use Stage 1 valves. the primary problem with big valves in the 430 / 400 was shrouding.
yes, it would be good practice to block all extraneous oil passages. you should compare your heads to the block deck and gaskets just to be sure. this page says you shouldn't use the steel shim gasket for an old head onto an oval water passage block.
The way to crush the bourgeoisie is to grind them between the millstones of taxation and inflation.
Government schooling is about "the perfect organization of the hive."
H.H. Goddard, Human Efficiency (1920)