TFMSS we did it very similar to yours as far as cutting the crossmember. My dad took it upon himself for cutting/fitment (I used 3"x4"x.125" I believe), he's a diemaker so I trust him with a grinder 8) Mainly what we did for fitment was after the old exhaust was removed we held the section of channel up to the crossmember until it appeared to best line up with the header outlet/exhaust x-pipe and marked it. he then trimmed bit by bit until it just barely slid in, leaving a portion of the crossmember so it was intact. We took it somewhere to have it welded because we don't have a welder.