Okay, let me go down the rabbit hole on this one. In my opinion, assuming you're Quadrajet 4-barrel is in good shape and the engine is properly tuned, good points, rotator, cap, wires, spark plugs. Baseline A. Changing to an Edelbrock Performer and take your pick of carburetors, horsepower gain= zero to one horsepower.
Why? The factory Quadrajet and factory 4-barrel can deliver more CFM than the engine is able to take. Why is that, one may ask. These cars are "difficult" to upgrade the "weakest link", the exhaust. Best case "factory exhaust" was the RARE factory S-drivers side manifold. I'll put a link to Thornton's. These manifolds were VERY RARE from factory, basically 1965/66 Starfire only. Thornton reproduces it now. Next, using this manifold, you'd have to modify your transmission shifter linkage, not a big deal, for some. Before Thornton started making the S-manifold, I'd cut, rotate, weld the Toronado driver's side manifold to get away of the "rear steer" steering box that makes putting tube headers on these cars, WITHOUT major modification. That is the main reason the Edelbrock performer will NOT give you any performance, imo. Next problem, factory gears. I scrapped out 20+ of these cars in the 1990s. I found two 3:08 posi's, most have one wheel 2.78 ratio. "IF" you want to spend money (after factory tuning/rebuilding Quadrajet and engine) put your money into the exhaust with Thortons manifolds. I'm just trying to save you money and point you in the right direction.
I agree, the performer, especially with a stock cam is a weight saver. Send your Qjet to someone like Everyday Performance, to do a proper rebuild or pay for a later 800 CFM core built to your specs. Your factory carb is 750 CFM, there will be more gained going to a tuned later carb than swapping intake manifolds. Are these the divorced choke carbs? Easy to convert the hot air choke carbs to electric. Dual exhaust with the above manifold is the best bang for your buck, hell even a 260 responds to dual exhaust. Between that and a proper carb rebuild will make it night and day better.