IIRC, the 215 was the Chevy design, so the tranny probably is a true PowerGlide. The V-8 Pontiac two speed was the SuperTurbine 300.
If it were me, I would go with a 400 or a 455 if you want to keep it pure Pontiac. I would also go with a Turbo 400. I will warn you that you will have to do something with the crossmember. On the coupes and hardtops, you can just move the crossmember back and drill new holes in the frame. The convertible frame is boxed, so the crossmember mounts to tabs welded to the frame. Since there were no "long" trannies offered in 64-66, there are only tabs for the "short" trannies. You will either have to section and offset to the rear the center of the crossmember or you will have to weld new tabs to the frame. You will also have to come up with a set of 1st generation Pontiac V-8 frame mounts.
Or you could go with a Chevy V-8. I think the V-8 will bolt up to your tranny, but I wouldn't go much more than a lo-po 350 with it. But a 350/350 combo would go right in with the proper engine frame mounts (1st generation Chevelle frame and motor mounts).
Always had a soft spot in my heart (and head) for 64 -67 Tempest/LeMans/GTO ragtops. Had a 66 Tempest Custom with a 65 GTO 389 and a 67 GTO with a 427 Chevy. Also owned several coupes and hardtops. Wish I still had them all! Post some photos.
