beam?

[GS guy]

Rip, new needle bearings and bushings are available new - the problem being having the proper tools to change them. By the time to buy the bearings and come up with the tools, or pay somebody else to change them out for you, you'd probably be better off just getting a new beam and starting with that - unless the beam you have is in very good condition and doesn't really need new bearings, just a good clean-up and painting. ALA the trailing arms rotate freely and don't have slop in the beam (installed without springs) and the towers aren't rusted out, and it's straight, it should be OK.
I believe the beams sold with adjusters already installed are set up such that the "full up" position sets the grub screws at the OEM location, and any adjustment goes lower. It sounds like you want to position the adjusters mid-way - so that you can rotate the torsion leaves both above and below the OEM position. For that, I think you'll have to install them yourself.
I also believe 99.9% of the ready made adjustable beams are assembled with Mig welding, from the looks of the welds I've seen. That method should be fine for what you want to do.
Jeff