No, it wouldn't fit. Cadillac, Buick, Pontiac and Oldsmobile had their own bellhousings which were not interchangable with Chevys. If you want to run a Chevy transmission, you need an adapter plate, like this one: http://www.summitracing.com/parts/TCI-230001/
Where are you located? I have an extra Cadillac TH400 transmission.
You may want to look at the tranny. I had a Buick with a 350/350 and the tranny had dual bolt patterns. I fit a Pontiac 400 right up to it. This was back in 78-79.