The technocarb is only 750? I thought it was like 900-1000 CFM. The Impco 425 is right around 425 CFM and can seriously chock a large engine or a modded one that is built to take advantage of propane's octane rating. For the spreadbore are you talking a Thermoquad or Quadrajet type? tiny primaries and huge secondaries? Because that will make a difference over the holley style, it will use less fuel till you actually get into the 4 barrels.