Adapting the 4L60 to the GMC will eliminate the bellhousing motor mounts on the SM-420 setup. There will be no way to bolt the GMC/4L60 in without fabricating new mounts, at least at the engine/trans junction point, possibly altering the bellhousing crossmember for clearance for the transmission, and fabricating a rear crossmember to support the tail shaft housing of the 4L60. Plan ahead!
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!