@dano , no he didn't take anything apart, but I know he's been doing this for a long time. I do trust his judgement, but I just can't afford the money to change everything and stroke it out.
So that leads me to ask how does he know you need new ones? If its just based on his opinion of the strength of stock components then sure (and they are fine for your build if they are good) but you'd have to disassemble and measure things to find out if they are out of Spec. cracked or off in some way.
At this point ask yourself, do I run with the used short block I have or redo the whole things from the ground up from who is being represented as and back up as a quality engine builder? Just based on the piston information being possibly incorrect from what you we're told you we're bough I'd go through it all. Building engine isn't inexpensive anymore, do it right the first time. Whats his opinion on the heads?