GMC engines have size as first three numbers of the engine number on pad behind the distributor. Clean or scrape off crud, maybe hit with brakleen to see it.
The trans will fit, BUT, it's not synchro, and you will have to always double clutch, a PIA I think. I had a 51 F-6 like that, hated driving it. Never did learn to downshift w/out some grinding. The 48 and up have synchro tranny's, diff model than earlier ones, but better shifting. Get the later trans, or leave what you have in truck. As you noted, speeds are about same with either. These are 50mph trucks in those days. Drive slow, enjoy as is !
