Before anything else, I'm aware this thread is more than 4 months old.
You are encountering the usual problem with the '76 to '81 catalytic convertor subframes, not to mention the mount locations on the unibody.
As you would suspect, these changes were made to accommodate the new for '76 emission control systems.
I believe that if you cut the trans mount bracket off the crossmember and re-weld it to the back side of the crossmember, the original bolt holes should line up. I remember doing this back in the "put a THM 400 in it!!, days. The THM 200-4R and the THM 400 have the transmount location the same distance from the front face of the transmission case. Some cars will require an offset mount. Unable to tell you if these are still available, or even what vehicle we used to source them from. I'm sure other members can fill in some of the blanks, here.
One more thing, if you go this route, measure everything before doing ANY cutting. You may also find that transmission pan removal after performing this mod may require raising the back of the trans and removing the mount, and possibly even the whole crossmember.
Don
