The first 340 I ever built I had a comp hi energy 268 hydraulic in it with comp lifters, with a decked block and milled heads - nothing fancy, just enough to make 'em straight and flat, and stock rockers. That motor with stock rockers would break up at 4500 rpm due to the lifters pumping up and the valves not closing. I added 273 rockers and it revved to 6000 no problem. If you've decked the block or milled the heads I'd run the 273 rockers. Either that or read up on shimming the rocker shafts if you run into the issue I described.