A Holley 650 carb is not too much, the Autolite 4100A carb is rated 635 cfm. The carb on the Cobra Jet engines, the 4300D, is rated about 750 CFM. Replacing the 4100A carb with a 780 Holley (no other changes) was good for 11 horsepower and a few extra rpm. Valve float or the ignition were the ultimate limits to the maximum engine speed.
A word of caution, I hope your mechanic is replacing the OEM crankshaft damper ... this is a serious issue.
A word on intake manifolds too. A dual plane 351C manifold combines the center two cylinders from one bank of cylinders with the outer two cylinders from the opposite bank of cylinders. This spreads the pulses in the manifold as far apart as possible, thus improving mid-range power. Based on that definition, no Offenhauser intake manifold is a dual plane manifold.
There is one Offenhauser manifold that some people like, known as the Dual Port manifold, it is available in several versions with different carburetor mounting pads to accommodate just about any carburetor you may wish to use. (#6013 for Holley 4150/4160 carburetors). If your Pantera's engine is equipped with any other Offenhauser manifold other than that one, you'd be better off replacing it.
Which path to follow really depends what's most important to you, originality or squeezing a few extra horsepower from the engine. If 300 bhp is enough for you, then acquire and rebuild an Autolite 4100A carb, mount it on an OEM iron manifold (casting #D0AE-L), and move-on to the next project. I agree with previous comments that originality is bitchen, and it contributes towards maximizing the value of your Pantera. Keep in mind some early push-button Panteras were equipped with an aluminum Ford manifold and 600 cfm Holley carburetor.
The OEM iron manifold can be modified for better performance & a bigger carb if you wish to invest money in it to do so. But you don't necessarily need to use an OEM iron manifold. You can use an Edelbrock dual plane manifold, grind the name "Edelbock" off, and paint it blue. Or you can use a Blue Thunder manifold and paint it blue. I kinda like that second option.
There are a few problems with a 4100A Autolite carb that may make you reconsider using one:
(1) they are small. They start choking the engine around 5000 rpm.
(2) they are known for warping the castings when being over-hauled.
(3) few people these days know how to "tune" them.
If not, I can give you the name of a tuner in San Diego California that is adept at working on the Autolite carbs. Or you can opt for a 600-650 cfm (or larger) square bore carb made by Holley, Summit Racing, Quick Fuel, Demon Carburetors, Edelbrock, etc. If the carb is hidden under the large OEM air cleaner assembly, nobody will know which carb the engine is equipped with other than you, or perhaps a potential buyer.
It would be possible in this day and age to build a 351C that is externally OEM in appearance, have good drivability, possess 100% reliability, and make 420 to 450 bhp. But all bets are off if short-cuts in budget are taken, or if a mechanic inexperienced with the 351C does the work, fighting experienced advice around every corner.
Mechanics can be a hard headed group of people.
