[Mr70]

I remember the Budd brand frames were a single piece tunnel bottom with channel rails welded on.Very distinct.
A.O. Smith brand used a double tunnel bottom with welded channel rails.
I took pictures of these examples awhile ago,but I can't find those anymore.
I've seen the Budd brand frames used on 1970+ Taxi cabs and 4 door Chevelles,as well as the Big Passenger cars.
Other common Frame Manufacturers back then were Chevrolet,Pontiac and Parrish Pressed Steel.
I think Budd still makes frames today under the name Thyssen-Krupp-Budd for the Chevrolet Trailblazer,GMC Envoy and Buick Rainier.