Body removal from chassis

I am really confused by the OPs reasoning. Is the non convertible chassie he has rusted out? That's the only reason I can find to replace sheet metal frame rails etc.

As for streighining, is it going to be a full race car?

You could add stiffining plates etc.

You could cut the bottom of the frame sections off and form a frame to slide inside the sides of the remaining rails.

Hey dana67 the reasoning is here in our nanna state of Australia we can not put modern motors in pre 1990s cars unless major structural changes are done. I was refused by the government body to carry out my build, they require full chassis to meet engineers requirements. So my 2 options are using this other chassis already done and engineered or have my chassis converted to full chassis to engineers requirements.

These rules have only been changed 18 months ago an make it harder. New rules for pre 1990s cars are 180hp per tonne so I have a 6.1lt hemi gen 3 ( not LS ) to go in it. Just seeing how difficult a body swap would be prior to purchasing the other chassis.
Both chassis are solid an no rust.
I hope that cleared it up

Cheers