I think the only major difference is:

the lack of side motor mounts before '58,

the 62-67 novas had a difference in provisions for clutch linkage, and a front oil sump bracket, (they also have a different bellhousing for standard trans),

and the "big one", that 400 smallblocks are externally balanced, so they use a different front balancer, and flex plate/flywheel.

If I got any of these wrong, someone correct me.