Slant Six has No power. will not accelerate

Owned my 68 Valiant for almost a month now. Since I've had it it has had a serious issue with power under normal acceleration. It starts fine and idles fine but does shake. But when I put it in gear I have to barely push the gas pedal down or it will stall on me. Anything more than say a quarter push down on the peddle will cause it to stall. Recently I've changed the spark plugs and fuel filter. Today I just tested the fuel pump and there is good suction from the pump inlet and fuel does come out of the outlet. I'm stuck on what to do next. I'm thinking maybe something with the distributor or the vacuum advance diaphragm. Any help and advice would very much be appreciated! Thank you in advance