Disappointing dyno results

Your cam is killing that combo, check out the duration on a stock 340 cam.

68 340 4 speed cam:

276°/284° duration
.444"/.453" lift

Int open: 26° BTDC
Int close: 70° ABDC
Exh open: 78° BBDC
Exh close: 26° ATDC

Overlap: 52°

Per the 68 factory service manual...