Everyone has mentioned valve float and after you have experienced it you will look at ways of limiting it.For me I ran a stock 340,with j heads,isky springs and 653 Mopar solid cam to 7k(tacho works fine) with no float(engine did have a few extra mods but stock bottom end)What killed the engine was partly detonation and mostly bearing failure,ie stock sump trying to supply the engine oiling requirement at those revs.If you want to rev the 340 and make it live you will need to look at some aftermarket parts and go into block/valve train prep.