Reply to "650 Double pumper vs 750 vacuum secondary"

In carb swaps, it pays to go to a real expert- if you can afford the cost. A box-stock race carb will always be rich- the mfgr figures you won't blow up an engine going rich so his warranty costs drop. And with a street carb. you have no way of telling if it matches your cam, intake & exhaust, or is jetted for your area. My home in N. NV is at 4750 ft of altitude; L.A is essentially at zero. Denver is at 10,000 ft. So where should they jet a box-stock carb (many of which must meet emissions specs)?
The real fix is to go find a tuner that is intimate with the needs of a 351-C, and pay him. The result will be more power, a closer-to-optimum mixture for your area & driving style, with a host of little internal tweaks inside to do this, not just a jet swap. We all make noises about "mileage means nothing; power is all"- but it DOES. Its annoying to be on a run with your friends and you are the first one to start looking for a gas staion....
A tuner carb will add 50-100% of the cost to a mail-order carb and it will be worth it- and this is from someone who has remachined carbs: moved jets, bored out and recontoured venturis and replaced throttle plates with larger ones. The months you'll spend fooling with jets, secondary springs, metering rods, etc. with a cheaper box-stock carb could be spent driving your Pantera with pleasure.
FWIW, I'm very pleased with my $750 tuner-Holley double-pumper 700, extensively reworked by an expert. Power went up while mileage ALSO went up 2mpg, over a 600 vac-sec, a 650 double-pump and a 750 vac-sec. These last three are for sale cheap to anyone who wishes to prove this wrong.