Make certain to take him a copy of the GM specs and don't let him deviate from those. Especially: piston to cylinder clearance. Take him the pistons marked 1 through 6 and have him match each piston to each cylinder so there won't be any questions. Check ring gap also. I've seen some rebuilt engines that were bored & honed so loosely and poorly they were going to run like they had 20~30,000 miles on them the first moment they started. Get the camshaft checked and reground by a specialist if needed (Schneider Cams in San Diego for example). Get new valve lifters...solid lifters. The rear crankshaft sealing surface is really important to making sure you don't have a leak there. Have him install a Speedi-Sleeve back there if needed. Use the rubber rear main seal. When the crankshaft is re-ground make sure the GM specs are met...try for .001" crank journal to bearing clearance. On the camshaft gear, try also for .001" clearance between the thrust plate and camshaft. Get the top of the block and head checked for flatness and re-surfaced if needed. He should know all of this but some machinists may not think of everything. If you're having the head re-done, use bronze valve guide inserts and check the old valves carefully. If the head or block must be resurfaced very much be sure to recess the valves in their seats, too.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end