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...
