Building a motor, where to start?

[Goat Blower]

Actually, you have to know if the block is .020" over already, so you do need to measure it, but you can do that yourself with a cheap caliper. Most aftermarket pistons for DSM's come in .020"(.5mm) or .040"(1mm) although I've heard of going up to .060" for that mythical 2.7L build from a decade ago.