The fluid coupling (no, it's not a torque converter) from the Chevy will probably interchange with the GMC transmission. If it's in good shape, that will probably solve the fitment problem when the GMC trans is bolted to the Chevy engine. Be aware that the runout between the bellhousing and the crankshaft centerline is extremely critical when the hydramatic is mated to the engine, so much so that Chevy matched engines and bellhousings at the factory on H-M equipped vehicles.
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!