Whats the deal with the KB 243's, good or not?

Yes, the KB slugs will have a guide in/on the box as to what to tap the rings at and for there useage. That being said, they also have a gap size for N02 useage. You can go to the Kb web site and get a heads up on what is needed.

I myself am almost ready to assemble a 340 with those pistons. I’m waiting on a little warmer weather.
Excellent