To "quesstimate" HP you need more imfo, compression ratio, cam lift (for sure), cam duration, intake (dual plane or single plane) just for starters.

The studs on small blocks are usually 3/8", so they would require 3/8" nuts. BUT some builders add 7/16" syuds and for these you would need 7/16" nuts. The firs thing to verify is if the rocker fit the studs properly.

Good Luck