That trans with the aluminum top probably isn't a Muncie- - - -more likely it's a New Process 435. Some GMC's came equipped with them, but they were mostly found in Ford and Dodge trucks, with a different input shaft and bellhousing bolt pattern.
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!