I'd be interested in how JB did his too. I can say that the earlier A-bodies (64-67) are a bit tighter around the front crossmember. The later frames had more of a flat area on the back and top side for pan clearance, the early ones are more of a "v" if that makes sense. I went with the F car pan, well for one I had one, and second it's shallow on the pickup side for ground clearance, and has pretty decent inner tie rod clearance. The biggest reason I notched mine was to get some clearance between the back of the passenger side cylinder head and the firewall. Some guys notch the oil pan instead for the same result.
Here's a couple pics of my notch:
