There is nothing wrong with a 3/4 ton or a Pontiac engine. It may be geared too low for a daily driver. I don't have a GMC book but the same year Chevy would have a 4:57 rear end.
Also, I think GMC mirrored Pontiac with engines so a 58 could be a 347. You could use your 4 speed trans with a SBC with the Chevy bellhousing. You would also need mounts, exhaust, and other incidentals. It seems like a lot of work & expense for little gain.
Fred
