Just for reference, I used 2 x 3 x 0.125 tube with the 3" being verticle on the boy's 72 duster. I went all the way back to my MP inboard spring kit box by only cutting out the TOP 2" where it interferes with the rear subframe ( Like ken is asking), so I overboxed the original subframe. What is strange to me is, with the above done the floorpan DID NOT need to be cut open, as it perfectly butted against the floor when it bottomed against the original frame rails. I am wondering if my framerails are taller than some others that have done A bodies? Anyway, my framerail has not been put to the test, but it was nice to not have to cut sheetmetal. This also allowed welding the rail to the floorpan (where the ribs allowed contact to be seen) all the way foreward to the higher section for the front seat. I think that my car was built different than many others, especially since I used bigblockdart for reference, and If I recall they had to cut the pans! Maybe I got metric tubing-LOL Hope this helps Allen