Reply to "Manifold Question - Performer vs A331 Torker"

Gotta agree with Mikael. The Holley 6619 vac-secondary 600 cfm carb was at one time the most popular bolt-on carb for Panteras that were actually driven any distance. Good power up to maybe 5500 rpms, terrific throttle response and 21-22 mpg. It will run out of breath above 5500 no matter what intake you use but was the carb of choice for autocrossing for Mustangs, Panteras and 'Vettes.
With a 750 Holley vac-sec, figure on gaining 20 horsepower, losing some throttle response and dropping mileage to 17 with no other changes. A 750 double-pump is a race carb and will usually drop your engine into the 12-13 mpg range with little extra power. They desperately need tuning on your engine to run right. Double pump carbs are calibrated rich out-of-the-box.
The carb size and type you need depends on how you drive, where you drive and how much you can afford. On most really hard-driven Panteras, a tuner-modified Holley 700 double-pump will be ideal up to around 650 bhp, and will cost around $750- nearly double the out-of-the-box price. My 700 Holley from CC in Reno gives great power AND 20 mpg. It has lots of stuff inside it that Holley never saw. There may be other combinations of a similar size but I'm satisfied for right now.