Keep hearing about frame twist with 440s

Concrete isnt ever dead level. Unless you payed big dollars or it was a gubmint contract. Thats why you go get 4 cheapo spirit levels from harbor freight, and tape them in place as follows one across rear transition pan with trunk mat out, one across the top inner lip of the radiator support, and one on each rocker panel in door jamb with the sill plates off.

Your jack stands should be on the framerails just forward of the K frame, and at the rear shackles. Jack the car up and place it eyeballed as close to level with the stands in these 4 spots. Now using aluminum or steel shim stock, slowly stack the shims between the Jack stands and the framerails until all 4 bubble levels are level. Now your chassis is level and square. At this point weld in your floor pan and subframe connector.

If you want to be sure the chassis is square when you weld it up, this is the way to do it. These cars are all flexi flyers.