Duane, I did that when I had my motor out last winter. Now this is in a Gen II car so YMMV ... You completely remove the bolt-on bracket that the toploader mount is attached to. Just aft of that, two slotted holes for the GM mount are in the welded frame, in the crotch of the x-brace. You must use the McLeod 5/8" transmission adapter to position the TKO over the holes. Thus you should use a
long input shaft TKO for an ERA 427.
Installation tip: jack the back of the motor up about 1.5" (loosen motor mounts, you may or may not want to undo the headers, I don't). Stab the tranny in and bolt it up .. then bolt the GM mount to the TKO, then drop the motor down and the mount studs will fall into the slots. If you don't do it in this sequence, you'll have trouble with the back of the trans housing hitting the frame before you can get the input shaft aligned with the splines in the clutch. I would support the tranny underneath with a jack or a muscular assistant
Edit: I DID have to move the shifter hole. The reverse on the TL was way down and to the left. then I had to move it again twice when the accordian boot kept springing me out of gear. Finally gave up and went to a leather boot. I made an aluminum plate for the new hole, riveted it in underneath and filled the surface depression with Bondo.
Sam