Pain in the Tailshaft!

I wish I had an answer for you, especially because the exact same thing happened to me. I bought a '79 stepside dirt cheap that had pretty much a brand new 318 in it. Cam, 4 bbl, headers, etc. 727 looked like it was done at the same time. I decided to swap engines with my '77 shortwide (both 2wd), also a 318, but getting pretty tired. Motor and 727 direct from the'77 into the '79, didn't change mounts or brackets... and the trans mount hole was at least 3/8" too far forward. Checked everything my buddy and I could think of, and finally gave up. IIRC i slotted the trans mount a little, and used a smaller diameter bolt between the mount and crossmember. Traded the stepside to him shortly thereafter, he's still running it that way.