The floor pans are only a part of the body floor structure. The body is secured to the frame with a series of bolt fasteners and rubber cushions. This link is for a pad kit on flea-pay and it has instruction sheet. Bolt kits would be available separately. https://www.ebay.com/itm/1938819927...MInei0k8Wr-wIVkhXUAR2BzgpTEAQYAyABEgKRdfD_BwE
I replaced the rubber mounts on our '41 fordor but don't remember the rect. pieces. I had to shuffle the rubbers around a bit because they didn't seen to fit according to the diagram.