Communication is truly key. The best companies I deal with in the car repair, home repair, "any type repair" space are just flat out honest. Upfront pricing, and a well detailed invoice go a long way. Big bills sting everyone but communication before hand goes a long way.
My biggest concern...
Where is home base for you? IMO you are better off towing that thing to a shop that specializes in. 6.0s. The shops that specialize in 6.0s will actually save you money in the long run because they are just better at diagnosing problems.
Domestic Diesel in Chino is one of the best.