Are you sure he's a mechanic?
first place to start is the engine number. After so long it could have been changed, rebored...anything. Once we know that, then we can advise you what engine it is. The engine number is stamped on a little "plateau" at the front side near the radiator end.
Next is has it been rebuilt and bored out......
Do the existing pistons have any markings on them (like +020, +040 etc).
Regarding the head, under the rocker cover on the "top" of the head, there is a number cast into the surface (like 12g940 for example). What is this number?
I'm surprised he asked you to order pistons without knowing what the bore size was, or even suggesting a rebore to a larger size, back to my first question
Get those numbers and castings, then we can help shed some light on it for you.
KB