No, the bellhousing is not anything like a normal Chevy pattern. You've probably got a Clark 5 speed with the round SAE style bellhousing. The 401 is one of about 4 or 5 sizes of GMC V6 ranging from 305 to 472 or so. They are very heavy, very hard to find parts for, and horrible gas guzzlers. I've got a 401 in an American LaFrance fire truck on a 61 GMC chassis. It's got a Clark 5 speeed with direct drive 5th gear- - - -wish it was the overdrive version! PM me if you decide not to use the engine/trans combo- - - -I'm looking for spares for both.
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!