Take a hammer and hit the frame in various places. The sound of the knock will tell you if it’s solid or not but it’s a slim chance based on what when’ve seen.
Sorry I don’t think that frame is fixable. Drive south and pickup a truck that isn’t rotten or you will just spend money without getting anywhere. If the cab mount brackets are that bad I can only imagine the rear frame rails.
94-97 T-56 firebird is what you want to find and they are the same length as the 60E.
https://www.gmt400.com/threads/gmt400-4l60e-to-t56-swap-write-up-dialup-warning.632/
I use a motorcycle Jack for jobs like this, they work great. You can add plywood if needed and be sure to use a ratchet strap or something to secure the trans to the Jack...