I'm assuming your block if already at 0.030" over. Did you ask them about cutting/honing your block for 0.040" over pistons?

If I were going to buy a short block, I wouldn't pay a premium for a 383 over a 400 unit. That doesn't make any sense. They both have a 3.750" stroke for torque.

Who did the final assembly on your motor? If you, did you wash it to within an inch of its life and lube everything before you started bolting parts together?