Look up a guy named Darious, not sure if that's spelled right, he did the whole swap in a RR but tunnels should be close between the two cars. Had quite a few headaches with that swap, especially the driveshaft hookup. Car Nag has a three wing flange but the Freightliner van uses a four wing so it can move up and down with the slip type u-joint.
Darius did the swap in a 70 GTX. I'd suspect that the trans tunnel in the B body is bigger than any A body. He did also switch to an RMS setup and cut the torsion bar crossmember and floor to clear the transmission.
