G-bodies used different crossmembers depending on the year and/or
transmission. Most turbo 350 and turbo 200 equipped cars used a "diagonal"
style crossmember. When the turbo 200-4R became available, A "straight
across" style crossmember was used. Some early 80's cars can use ONLY the
diagonal style crossmember. Later years had mounting bosses on the frame
to use either style crossmember. The passenger side of the crossmember
is designed to clear the stock cat and single exhaust pipe. On the drivers
side there is no clearance forcing you to bend the pipe under the crossmember,
which is not good for exhaust flow, or road clearance.
You can make the modification yourself using a chop saw and (a MIG) welder. It is fairly simple to do. If you run 3" exhaust you will need room to run a large diameter tube without it causing ground clearance problems. Here is a modification you can do yourself and save some money.
1. If you have long tube headers installed you can "eyeball" or use a straight-edge from the header to the crossmember to determine where the crossmember needs to be clearance where the exhaust pipe will travel.
2. Remove the crossmember. And notch the crossmember and box it in with some flat steel.
3. Bolt in the modified crossmember, and run the exhaust.
In your area; it may be mandatory that your vehicle maintain all structural integrity; which doesn't allow for this mod. In this case, get the car saftied; and then do the crossmember swap. Remember; the crossmember IS supporting the tailshaft of the transmission; so adequate support under the transmission will be necessary.