It may be easier to modify your original crossmember for duals. I used a piece of a crossmember from the junk yard. Get a piece about 10 inches long, cut in half to make a channel, then taper and the fold the ends down. Make it to fit between the original tube and bottom of the floor pan with about 1/8 of an inch minimum clearance. Weld the piece to the top of the tube, and weld any open edges. After welding it to the top of the tube, cut the bottom of the tube up to the bottom of the top of the original tube making a hump in the crossmember. Cut the tube at an angle to match the path of the exhaust pipe under it. Fill the open ends of the tube with parts of the left over tubing by cutting to size, and welding the pieces in the ends, sealing all edges. Mine is done that way for my 91 OCC, and yours should be almost identical to mine. You do not need a jig to hold it, because it will not warp if you follow my instructions above. You can buy ready made crossmembers for about $150-250 if you check around.
The 94-96 crossmembers require you to flatten the top of the left pipe to go under it. You can make your OEM crossmember to allow the full height pipe go under it. Your tubular one is lighter than the 94-96 as well.
1991 OCC 461 (.030 over 454) BBC, 3.23 posi, flash to pass, drop spindles & springs, Impala rims, Hydroboost, Recaros, MOMO/wood SW w/QR, custom wood shift knob, Pioneer DEH P77DH
1992 OCC now with 5.7 tbi, DEH P77DH
For a parts list, check https://www.impalassforum.com/vBulle...ion-parts.html
one of the diff between the '77-'93 frames and the'94-'96 is the tranny x-member. On the pass side the mount for it is about 8" farther back than on the earlier cars. The squared off part extends back at least 8"