We know the stock 340's were under rated @ 275. A few years ago, Dick Landy rebuilt a 340 to stock specs. only .030 overbore. With the AFB manifold and carb, it made 315 on his dyno. NHRA factored the 340 @ 310 when it first came out. The six pac's were factored @ 330. The '71's were also factored @ 330. I too believe that your cam is hurting the heads from breathing up top. Something in the 218-228 @.050 with at least .480 lift range would kick this combo to 350 horse range or more. It can easily make more than 350. 400 horses is to easy with these amazing engines. THE best small block ever. Good luck.
Terry.