It is perfectly legal with SFI to weld chrome moly to mild steel. But, all the tubing involved has to meet the SFI spec.

My question is why would you weld chrome moly to the original sheetmetal?

As far as the basic cage there are requirements of what it welds to. Box tubing must be .083" thick. Round tubing also has to be .083 for chrome moly and .118 for mild steel. Your stock unibody frame rails are not .083" in all places. When they are that thick it is usually due to layers of sheet metal. You would need to penetrate the layers. If you are welding to the floor, use a 6"X6" plate. The rear frame rails for the rear support bars can be done several ways. The best is probably a 6"X6" bent to wrap the rear frame rail. Brake the corners down to make a diamond formation. This will provide the strongest support without compromising the strength of the frame rail. The front frame rails are thick on top but layered. If you choose to weld on top of the frame it is tricky to penetrate the frame rail without burning the tubing up. I have seen several approaches.

If you want your cup on your tig torch to live welding on Mopars? You better leave the tig alone when welding to the stock sheetmetal. I choose to mig a plate to the stock metal. Then tig my chrome moly to the plate. I have seen extremely weight conscious people weld a heavy bead with a mig, grind it flat, then tig the tubing to it. I feel that is a wast of time. I don't know why but my tig hates stock mopar sheetmetal. I have tried everything from 64-97 and got the same results. Chevies and Fords seem to do just fine.

I just got through welding a roof skin on a Dakota that the guy drilled all the way through on the spot welds at the rear door corner. This was a criticle area as the headliner is in the truck. So I had to tig it to not catch everything on fire. When I got done I had to rebuild the torch head. Just another $20 on the bill. LOL

Leon