I installed an OOTB Edelbrock on a 390 Galaxy. The guy couldnt understand why it would barely start with tons of pumping and high idle. The friggin vacuum brake port on back was wide open. Sounds like you have a vacuum leak. My 273/4 would idle down to 600 with its original 4bbl and if the fan wasnt hooked up, you wouldnt even know it was running. Put a vacuum gauge on it! Also confirm the choke is working: choke closes, step up cam engages and throttle is cocked open a tad and of course timing is correct. That 273 is a tight motor.
Oh yeah, clean the crap out of the lower body. A good soak and compressed air blow out of the passages.
