What you can do is get the part number off of one of the pistons he took out and have him give you the part number of the pistons he is putting in. Then look up the application for yourself and you'll know exactly what you're getting.
Also, is he honing the block and installing new rings? While the engine is torn down that far, you might as well put in a fresh set of rod and crank bearings. Pulling the main caps is only one more small step at this point. I would want to visually inspect the crank for wear, and I would insist on seeing everything. If he's legit, he won't mind showing it all to you for visual inspection and to research any part numbers. Keep in mind that engine bearings, rings, oil pump, and even polishing the crank are all relatively cheap at this stage of the game. Good luck!

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'67 Mustang Coupe
424W-KB Forged 10.2:1-AFR 205's-Victor Jr.-Mighty Demon 825-Comp Cams hyd roller.236/.242 .555/.576-Miller rockers-Hooker Super Comps-Gene Hughes AOD-Broader Performance valve body-3.55 Posi-4 wheel discs