Some of you may remember POCA articles from a decade ago- a bright yellow '73 GT5-s clone that I got in my shop to 'straighten out'. It had so many issues that I did a couple of articles on the 58 problems I found. On that engine, a vac-sec carb simply Would. Not. Work. It came in with a brand new Holley Avenger (vac-sec) carb. I could NOT get rid of a mid-range bog during acceleration.
Tried swapping springs, jet blocks and even float bowls (center feed vs end feed). I tried SIX more vac-sec carbs of various sizes and List Numbers, with nearly identical results. Finally I took the car to Mallory's local shop for some expert advice. The dyno tech took one look at it and said, "Holley doesn't mention it but all Avengers are CA compliant emission carbs. You'll NEVER get that thing to function like you want on a performance 351-C".
Based on his advice, I bought a 'tuner' 700 double-pumper from Bob Oliver at Competition Carbs, and bolted it on. Instantly, no more bogs, more power and better fuel mileage. His carbs have some 24 separate changes to the new carb as-received. It worked so well, I bought a second tuner-carb for our Pantera, and it worked better than what I had (I'd tried my old carb on the yellow car; no help). A week late we drove to 'Vegas for a Fun Rally with the new carb and got 22 mpg at an avg mph well above 70! Then Judy ran the track event and was all smiles at the power increase and drivability (900 miles trouble free round trip).
Over 40 yrs of ownership and two engines (first was OEM; the last of which I built) with the same Pantera, I have learned to defer to real experts in certain areas. My advice is to either spend every evening in your garage fiddling with your setup & going to bed smelling of 91-octane until you get lucky. Or buy a tuner carb and immediately enjoy the results. A third possibility is a late throttle-body-EFI setup. I have no direct experience but they are supposedly marginally better for power AND mileage than tuner carbs, for 3X the carb's price & complication. And are still not fool-proof on street or track. Good luck.