PPATT, after all the posts on this subject, I'll ask The Question: What kind of Pantera driving do you do, mostly? Hot-street, cruising around town, shows, autocross, some/a lot of open track, or ??? The carb you need to replace that leaky 45-yr-old 600 vac-sec will vary for each of the above choices. If you are a do-it-yerselfer, Pantera owner Michael Haas wrote a 91-pg self-help Holley tuning guide ("Tuning Made Easy") in 2012 that is easy to follow and gives good results. I recommend it; at the end, you WILL understand carburetors better. You will also smell of high-test for a week. If you are not a DIY guy, buy a tuner-carb of a size that agrees with how you drive, and phone (NOT e-mail) the seller for his invaluable advice before you order from him.
Option 3: why buy another carb -unless you want to keep a traditional (but not stock) look? A Holley Sniper EFI is in fact a complicated DIY conversion (with on-line guidance), and while you will likely not increase power, your mileage AND driveability will increase. EFI prices are coming down to rival that of tuner-carbs, too. I also suggest you NOT buy used- either a tuner carb or any EFI conversion. The builder's advice is invaluable and it is included in the price. People who invariably buy cheap/used off E-Bay, then spend hours on the phone with a good-hearted expert (for no cash to support his l'il business) while trying to straighten out their low-buck 'solution', rubs me the wrong way. Good luck, whichever path you take.