Chevy and GMC engine blocks share the same bellhousing bolt pattern, so that's not a problem. 47 was the break point between the crash box and the SM-420 transmission on most of the trucks, so it's possible you've got the later model transmission. Does your shifter have a reverse lockout trigger? If not, you've got the SM 420. You'll need to fabricate a bracket to move the front motor mount back a couple of inches and do whatever adapting the exhaust pipe and throttle linkage requires, but overall it's a pretty simple swap. Good luck!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!