Short answer, A/C compressors are the same for 4.3 V6 / 4.8 / 5.3 / 6.0 gas V-8 and 6.6L diesel V8 from 2003+. So it doesn't HAVE to come from a diesel GM vehicle. Any GM pickup / SUV / van with a gas engine from 2003 through at least 2007-2009 should use the same compressor. Try searching car-part.com in the Albuquerque zip code, I found at least 8 possible matches, from various GM truck/SUV/van with various engines.
I can never remember whether the Micro Bird is the WorkHorse chassis and the Mini Bird is the Express / Savana chassis, or vice versa. Is yours a cutaway van cab or one of those Picasso-looking Step Van clone deals?
I seem to remember another member recently had similar issues with the same package (tried to remove a perfectly good system and could not reinstall the belt -- DUH...)
In trying to help them solve their issue, I found there is some fabrication and finagling required to make the Dorman part work. In the even that a good used compressor cannot be sourced, I've noticed most GM setups can continue with just a swap to a shorter belt, not sure if this one can or not. I did that once on exactly the same problem with a 3.1 Lumina and it worked like a charm. Compressor was seized to the point the engine could not crank. Shorter belt and it started and ran fine. Serpentine belt part #s go as follows:
338K6 = 33.8 inches, 6 grooves
724K5 = 72.4 inches, 5 grooves
Some Delco numbers put the grooves first, then the inches...
6K598 for the Lumina, for example (I believe it was 59.8 inches)
On the Lumina I mentioned, I think I used a belt that was about 11 inches shorter, as memory serves, which would have been a 6K488. But look at how your compressor is mounted and how the belt routes around it from the previous and succeeding pulleys to determine if a shorter belt would bypass it without issue. If it will, measure the run up and over your compressor from the closest pulleys, subtract that number of inches from the first three numbers of your stock belt part #, and it should work.