I'm pretty sure the largest six cylinder engine in a 64 Nova was a 230 cubic inch model. Not to say that someone couldn't have swapped a 250 into the Nova.

The Chevy bell housing should mate to the GMC trans.

The motor mounting arrangement is completely different, with 1 front mount and 2 side mounts on the GMC bellhousing vs. 2 side mounts on the 250 engine and the expectation of one transmission mount on the original Nova transmission. So with the Nova engine and the GMC tranny, there wouldn't be anything holding up the rear of the engine or the transmission.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.