On my '73 Duster we cut the rear frame clip in front & back open, used (been a long time) I "think 3"x2" mild steel" And opened the front frame clip and drove it all the way through as 1 piece. Then welded everything closed and around the new connector. Welded everyplace that touched the floor. You could tell a difference as soon as the car was started, very solid. Opened the floor & welded the cage bars to the connectors. Not street driven anymore.
Took a '67 Coronet Stock eliminator car, but used C/M tube and did it very similar. Both cars are very rigid now. Don't remember the dia. of the tube, but it just did go in with some "inspiration" while it was up on a lift. Think it took about a days worth of labor start to finish.