Need help from Tuning Guru's

I bought a old 77 p/up with a factory 440, thermoquad. Was a plow truck for the neighbor and rarely run. I'm having trouble figuring it out: idles fine, starts OK, new fuel pump and it checks out good with both flow and pressure. You nail it and it revs just so high and starts spitting and sputtering around say, 30 MPH in first gear (3:55's so it's not reving very high), I think it might be overfueling but not sure. New gas, I disconnected the secondaries @ the throttle plates and tied them and the choke open, Still does the same thing. I hooked up a vaccume gauge, 16" @ idle (I'm @ 8,500') it goes close to zero @ wide open throttle and stays a the same place when you have it @ WOT. What would the reading be on the vacume gauge say if the exhaust was getting plugged? If the exhaust was getting plugged would it make the exhaust black @ WOT? The carb has a rebuilt tag on it, though I don't know how old it is or if it ever worked OK on the engine. Any help and advice would be greatly appreciated!