I bought my Promaster from Car Culture, a dealer lot with attached mechanic shop located on 82nd Ave. They buy these rigs wrecked and resell them with reconstructed titles. They are a family owned local shop and not a chain. When I purchased my rig, I asked if they were open to mechanic work should I need some and they were. They may be worth contacting and it seems to be a huge portion of their business to rebuild these things so I imagine the mechanics are pretty familiar.
I tried looking for a link to provide but Google says they are temporarily closed. I work in the area and drive by the location frequently and it appears open and in business so your mileage may vary.