Stock block limits on a 340 with nitrous?

If it's filled and you have the 2 bolt steel caps you should be good past 700. I know of a couple non-filled that push past 650 on spray. A lot depends on the quality of machining on everything and keeping the tune dead on. Rich is almost as bad as lean. So honestly it's very subjective & it will either hold, or crack. Run it and tell us what happens.