I'm going to my local steel supply company and purchase some steel square tubing for (69 Roadrunner) frame connectors. I did the same thing on my Challenger, but this time I was thinking about getting 1x2inch tubing instead of the 2x2 I got last time. Reason for the 1x2 is I won't have to go into the floor, but I'm not sure if the 1x2 would be rigid enough. What do you guys think?

2X2 fits perfectly under the floor on 69 B-body. Just cut a hole in the front of the rear frame rail, slide the 2X2 inside the rail and then up against the back of the torsionbar crossmember. Without a doubt the easiest weld in frame connectors ever.

Not so good on an E-body.

Use a minimum of .120 wall tubing. Thats just shy of 1/8"
I used 3x3 for the connectors in my FrankenDuster. I made a template of the floorpan out of cardboard and traced it onto the tubing, then cut them. Mine were welded to the front and rear frame rails AND to the floor. I spread seam sealer on all the edges, then sprayed rubberized undercoating on the entire floorpan. It looks as if it left the factory that way.

I have a piece of 2x2 .120 left over from my e-body I did and I'm using that for refrence. I cut out the holes in the rear frame rail last night for the 2x2 that Gomango mentioned. I'm gonna try and get the tubing Monday. Thanks for the help.

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I think mine are 1x2", I think is plenty for a street car. That is the size Direct Connection offered in there connectors. I have them on my car and it is plenty stiff, way different than my black 74 which don't have them. Like you said they fit best when not cutting the floor.

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Yep 2x2 doesn't fit on E-bodies like it does on B-bodies. The rear frame hangs lower below the floor on the b-body allowing the 2x2 to just fit.

USCartool.