EFI for my 65 Barracuda -- 2bbl or 4bbl

The 2-4bbl thing was the amount of vacuum the venturis could pull and still be an active jetted carburetor. The 4bbls was just 2 more barrels because they couldnt make a 3" primary throttle butterfly and still have any venturi vacuum to pull fuel. The atomization of fuel was the deal breaker in making larger venturis, they just couldnt do it efficiently. Enter the 2bbl TBI. Massive throttle bores because the atomization was done by the injectors themselves, (think of the flying toilet) the bores were now just control valves. There is still a limit on tip in (off idle) control, but the area of two 1.95 Dodge TPI bores (eg) is 5.72 in^ while the D-Darts 4bbl carb was just a hair smaller at 5.67in^. If you do buy 2 or 4bbl, look more at the HP ratings. Even the 2bbl TB EFI are rated up around 300 HP and if you want to go larger, its usually just a TB upgrade, cheaper if you go MPFI as you can get a bare Ford style TB for like $70 new. The bigger units as stated will support smaller displacements but the duty cycle of the large injector will get very small and that are not efficient at their minimal flow rates. Buy EFI for HP, not for CFM.