I checked out GM truck center a few years back when I was in your position, and I wasn't happy with their storage and security situation. They are on a very small lot...
I did a lot of business with L&G in San Dimas, but that was about 10 years ago. They work on all kinds of custom stuff, and they work on classics as well.
They did my frame, body, and paint.
My truck was there for years, mainly by my needs since I was broke and living in a craptastic apartment in the bario.
By the time my burb was gone, they hated dealing with me
But... Louie (the owner) is a stand up guy, and he know's what he is doing.
Being that your truck is where it is in the restoration, I'd give him a call and send him pictures of it. He may take on the job.
Here is there website:
http://www.landgenterprises.com/index.html
Heck, I saw a truck at the recent Brother's truck show that was done up by L&G. So they apparently are still working on the old iron.
I think my burb was more work than they actually thought, and I gotta hand it to them they did not waffle on their estimate.
Be real carefull though on the shop you choose, since I got burned by a shop that was basically a bunch of ex con knuckle heads that did more damage than good.
It's a real pain to find shops here in SoCal that are classic friendly, since most of the shops are just parts changers. So anything needing skill and extra time, they won't touch it.
Good luck!
Oh, I did have a good experience with these guys when I lived in the SCV.
Sierra Paint & Body.
17528 Sierra Hwy, Santa Clarita, CA - (661) 251-9018
They don't do anything but paint and body work, so your assembly thing you will have to go someplace else. Just letting you know about them as a shop that isn't scared away by old trucks, and they were a just a bunch of cool guys to do business with.