what size BB stroker is best for race only with stock block?

Let's start with the goal.

Blocks are not the normal failure in a race effort. There's problems with crank walk, failing caps, rods before blocks crack in half.

The bigger is better applies here well, but it is not the end all or goal you NEED to have to start. The best thing about building a huge stroker is the ceiling it offers. You need not hit the ceiling first. There's a stair case for that high cathedral ceiling and you just take 1 step at a time.

Also a bigger engine (540 CID) see's less stress at 700HP than a small engine does. (383 CID) Less stress equals longer life. A stroker also makes there power at a lower rpm. Less stress again.

The draw back is expense. Also, generally speaking, if you build, OH let's just say a nice 540 cube engine but only give it well done factory iron heads, well then I say you might have just stuck with the 400/440 cube displacement.