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.
Thanks! I won't waste his or my time then. What engines and years will direct swap or where can I find that info? I've been searching all day. I found another motor and trans, (three on the tree) the guy says it will fit right in my truck, but it also came out of a nova. He says he knows without a doubt it would fix, but will allow me to return it if it doesn't.
