Destroked Small Block?

The biggest issue with destroking a 340 is the very tall deck height. A 340 already has a 1.850 compression height. As you destroke the engine that compression distance gets taller and that's bad. It means a heavier piston, and generally ring seal that can be marginal at best.

You can correct some of that by using a longer rod. I always wanted to do a .400 long BBC rod but back in the day it was tough to even get a rod manufacturer to discuss it with you.

Now, with billet rods out there you can get just about anything you want and it may be worth looking into.

The other option is finding an R block with a shorter deck...say a 9.000 deck block. I've seen a few here on FABO. That way you can use a 6.123 rod (I use 6.125 Chevy rods instead and turn the rod throws to 2.100 or if you are even more die hard turn them down to 2.000) and still keep the piston weight somewhat light and with the pin not over 2 inches down from the deck.