3b is correct. The '55-'59 GMC used a Pontiac engine in their V8 powered trucks, rather than a Chevrolet engine. You will need to find the correct engine mounts and bellhousing for a Chevy ('55-'67 will work, exept for '60-'62) to make the install work, GMC parts will not be compatible with a Chevy engine. The trans will work just fine, though. You will also need to source out the correct linkage for the first/reverse gear. The linkage is different between 6 cylinder and V8 for '55 (or at least it is on Chevy). There is a bellcrank that mounts to the bellhousing on 6 cylinder models that is not used/present on the V8 bellhousing.
As far as junking the old engine, what 3B was trying to say was to offer it to someone that can overhaul it, rather than just scrapping it. There's most likely someone out there looking for a good original engine for a restoration. Or even a rodder might want it. We'd much rather se that than it just get scrapped.
