what is the maximum amount

that can be milled from the top of a stock 170 piston? does anyone know?

According the Phil Irving, a non optimised cast alloy pistons can usually be dished or fly cut 3 mm or 125 thou. They were designed as a 6.5 mm dish piston, and then the sand casting was optimised. Ford in the post war to 1968 era left a lot of alloy and iron in its castings, even the thinwall 1962 onwards engines had a lot of redundant metal.

You'll have to confirm with a dial gauge, so its personal risk, but Ford were in the situation of saving alloy only after 1968, and that's when pistons got an optimised casting, so a 13 cc 250 1969 piston would be stamped 250, and a stock 6.5 cc 200 piston couldn't be taken out to 13 cc or you would find air!

good information X, i found a four main 200 crank, and by my calculations in order to use the pistons and rods i have now, i would need to shave .090 from the top of the piston to get the deck clearance back to stock. thank you my friend.