A 400 Crossmember was made for a BB Chevy so the holes for the tranny mount are offset to match where BB motor mounts sit them closer to the pass. side..
If you measure the one you have, if the trans mount holes are in the center it's a SB one and if they're closer to the pass. side it's a BB/400 one..
I don't know for sure but expect BOP X-Body cars used a SB style center mounted trans mount?.
I believe either style crossmembers will work w/ all trans applications as long as you elongate the holes to the side or center for whichever engine mounting position you have?.
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All my big 67 Chevys with 400 have the extra cross member mounts on the frame. They most have built one frame with the closer to the engine mounts and then added the extra set if a 400 was being put in