383 rebuild - kinda whimpy on the dyno?

Even with stock 906 heads breathing into headers, this combo should be more impressive. If the cam checked out fine, (once in a while they don't quite match the specs on the box) and degreed OK, I would be suspicious of the springs. He needs to provide you with a part number, and check that they match up with the box they came in. Idiots work in automotive manufacturing facilities too. What was the seated and open pressures? Were they too weak for the cam profile?