I'll swear by
United Diesel (link) in Springfield, VA. (No affiliation, but it's the kind of place I'd wanna work)
They don't do engine rebuilds per se (I've asked - haha!), but getting injection timing right, rebuilding injectors and injection pumps... pros.
I've been in there four or five times (and only twice for a job that pays them any money) and every time - every time - the guys behind the counter took the time to talk to my about my vehicle, the engine, the injection pump, blow-by, smoke, not priming properly, etc., etc. They educated me on what *should* be happening in my engine bay (03/81 BJ40 from S. Amer), what might actually be happening, and what I could do to fix it WITHOUT paying them. Sure, they were 100% willing to take my injectors and rebuild them. They certainly would have been happy to take a sizable pile of hundreds to rebuild my injection pump. But instead of saying "can't really help you without seeing it", or "you'll just have to bring it in", they took the time to talk to a diesel noob and make me smarter and better able to understand my own truck.
And when they did do work, it was top-notch and fairly priced. I cannot say enough about the staff of this place.
Hope to help,
TDC.-