those cab mounts wouldn't be too hard to fab from some square tube if you don't have the originals, you wanna keep the stock frame for reference dimensions until the donor frame is completed or you have a buyer for the original frame. if you have some fab skills (and I assume you must if you are planning the swap) it wouldn't take much. here is a link to the TCI frame page that shows how they made theirs.
https://totalcostinvolved.com/produc...4-chevy-truck/