Trade Street Demon 750 carb for 625 carb or other

i have the 750 street demon on my stock 340 and it runs great , i did have to do a little tuning (springs and rods) but after that it runs awesome, and pulls better than my themo did, although i could tune the thermo to make it better but knowing that any repair on the jet wells is not anything that will last i decided to stay with the street demon - maybe more timing would take care of the stumble , i am running 20 int and 36 total 55 when vac advance comes in