slant six cranks but won't start

I have a 1971 Plymouth Duster. It has been sitting in my grandfathers garage since 1991 and I just got it earlier last summer. I have redone the brake lines and was starting to work all the bad gas out by changing furl filters over and over. I had it running two weeks ago and changed the radiator hoses, fan belt, and antifreeze twice by accident. I then drove it back into the garage and now it won't start. It cranks and tries to fire if I hold the gas pedal, but never takes off. I replaced the carb, plugs, wires, starter, and now am out of ideas. I checked the spark at the plugs and it is sparking. Please if anyone has some help.

I know this thread has multiple problems from multiple members, so I quoted the original question for clarity's sake.
As a relative Mopar noob myself I'm curious as to why no one mentioned the OP's ballast resistor. Usually that's the first thing everyone says with these particular circumstances- according to the OP, it was running, he fixed some stuff, it was still running, then he drove it into the garage. Now it cranks and "tries to fire" but doesn't "take off". Just from what I've gleaned from you members more knowledgeable than I, isn't that a typical ballast resistor problem? Tries to start an immediately dies?

Just trying to understand, dont want to start any arguments :)