We've actually talked about the 421" using the chevy rods, more cubes with an even lighter bobweight but not sure the additional cost is warranted for what we're going for.

i find that 232/237 @ .050 on 108 +4 is a great all-around high-torque grind for naturally Torquey moderate RPM motors in the 400-420 range with a stick....gives it a nice big block type powerband with enough intake duration to pull to ~6000 along with a nice 'count-em off' idle. With a hydraulic flat tappet on these small-ish port motors it's almost as much of a concern to go too small on the intake lobe duration than too big.

Should be fun little project, been a while since I worked on one. Ever notice how much an AMC resmembles a Buick? It's not coincidence if you read a bit into the history of Nash-Rambler/AMC.