The difficulty with the caged nuts is not in finding them but in finding enough room between the floor pan and your transmission to get your welder in there. It will be pretty tough to weld the caged nuts in place with the body mounted over the tranny.
They were square nuts captured in a metal frame spot welded to the floor pan. You can get or make replacements that clip into a hole. You could call Walck's and see if they have something that would work.
During my rebuild I have never had any bolts on the floor pan where the transmission cover bolts. There are square holes, what did you use to assemble this. Speed nuts? Going to town tomorrow, would like to pick up what you all suggest. Thanks in advance. North