As far as the size of the carb, it depends on what you expect from your Pantera. Yes, the 600 (or so) carb will be leaving some horsepower on the table. But if for instance the performance venue you have in mind is autocross, the 600 will be more throttle-responsive out of the corners and will (maybe) give 20 mpg on the street. Going to a 750 was once known for adding 30 horses and losing 4 mpg while living with a soft throttle response.
As far as the intake, guessing is hopeless. 50 years ago, there were some pretty good Cleveland intakes and a few truly awful ones and you should figure out what you have. Some were marked underneath with ids. The least expensive way to be sure is to install all your parts and drive it. If you're satisfied, the questions are all answered. The most expensive way is to assemble the parts and dyno-test your combination. Good luck-