Mego Museum will definitely be a big help with these type of questions!
I did move a Retro Action Batman to a Figures Toy Company body with not much difficulty. The hands and head comes out easily enough when heated. I used the boil & pop method. The neck plug on the RA figures is a little rigid, but they soften enough to push them into the neck hole of a mego-like body. The suit is a little tight in some areas because the Mattel body is so different, but it will fit on another body type.
Here is my Batman swap (I still need to replace the fallen chest symbol). I also put a pair of Doc Mego blue shorts on him. He is on a spare FTC body I had swapped to a Type S body.