Fred,
Got in touch with the guy who did the conversion. He used the original 4 speed cross member. Had to move it back. His memory was that he used the original rear frame holes for the cross member and then drilled two more holes behind those - but he wasn't positive. Said that the original cross member also needs to be modified on the front portion to fit the Tremec. Just a small grind out to get it to seat correctly. The breather on the top of the transmission did hit his hump so he put some grease on it and then raised the transmission into place to mark where it hit then used a ball peen hammer to 'adjust' the hump to get clearance. He noted it was a tight fit, with maybe 1/4" of space in the tight areas, but he has put a lot of miles on his car after the conversion and has had no problems. He did not cut out any sheet metal.
Hope this helps.
Dan