Attached is an estimate of the bobweights, rotating and reciprocating. The actual weights will depend on the exact parts, these are just estimates for the dia and lengths needed.

The lower bobweight will help a little, but only a little perf; the main benefit is lower stress and higher durability. With a good high flowing head, 2.2+inch intake valve, etc. the bigger bore will help fill the cylinders better making more power. Think 400 block rather than 383 or 440. Or just get a megablock...