How street able are these 340 stroker motors?

It all depends on combination and expectations. If you are going for all out, 500+hp small block a 6pack is a poor choice. A well thought street build with a 6pack won't leave much, if anything, on the table. On 8k drag screamer, yeah.

http://www.moparmusclemagazine.com/...dyno_testing_small_block_intakes/viewall.html

agreed, I personally believe that assuming a well tuned six pack on any motor will cause a 20hp restriction at the very least is very pessimistic.