I'm in the process of retrofitting my car from a "Street Racer" to a driver. I am changing the intake to a Performer with a matching cam. The heads will also be changed from a set of 840 to 781 castings. Can you advise me as to a Carb with mechanical Secondaries vs Vac. I don't really care to drill a hole in the dash for a manual choke.

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Mark, I would highly recommend that you stay with a vacuum secondary/electric choke Holley 750 for a daily driver. I just tried and returned a full-tilt Barry Grant carb with mechanical secondaries and no choke. No go....hard to start,bog,stall. A properly setup Q-Jet is an excellent choice as is the new Edelbrock/Carter.

I ditto Pete's input, but would suggest you might want to stay at a 650 level...you'll be reducing the RPM that you will be producing your peak power; the 650 will more closely match your engine's configuration. (Use Pete's experience - Bigger ain't always better!)

Mark:
If the reason you want to change carb is for the auto choke Holley makes electric choke kits to fit any of their carbs.
A vac carb usually works out better on the street but a double pumper will work too. The DP carb will probably use more fuel, but if it's a weekend driver it may not matter.
I think a 700 CFM (4778) carb would work best for a double pumper or for a vac. secondary carb the Holley 3310 750 CFM is a good carb and reasonably priced. The vac carb can be larger because it will work on demand and not over carb your engine.
GaryR