new engine or stroker kit?

My first real suggestion to you is SLOW DOWN!!! Bouncing 100 different direction will get you nowhere fast.

You can use the stroker kit in your block with a fresh bore and hone. That scat kit is fine, just don't buy it balanced from scat. They don't even match the piston weights, rod big/small ends. Pay your local machine shop for a proper balance.

Almost every piece you currently have, balancer, flexplate, convertor, will work on an internally balanced stroker kit.

You can do another 340 on the cheap. If it was running good, quick hone, cut the crank, resize rods, fresh bearings/rings/gaskets and off you go.

Headers you have are fine for a reasonable build.