To fix long cranking after setting for awhile, the easy way to prime any carb is with a small electric fuel pump installed between the tank & carb, and a momentary- ON switch under the dash. The always-noisy electric pump bogs down slightly, telling you when the float bowls are full. The engine will usually fire instantly. Depending on which electric pump you use, you may need a check-valve in the line to prevent the OEM fuel pump from backflowing while it's off.
40 years ago the standard was, a 600 cfm carb gives better throttle response around town and better fuel mileage. Changing to a 750 instantly gives an extra 30-40 bhp, softer/slower throttle response and loses 2-4 mpg. A 351-C is a very easy engine to over-carb. This has not changed- pick what's important to you.
A vac-sec carb can (not always) run pretty close to optimum right out of the box, but no guarantees. A mechanical secondary carb is a racing unit so is ALWAYS jetted rich out of the box and may need an hour's dyno tuning by an expert to run well at both ends of the rpm range. Occasionally, an owner gets lucky and one will run acceptably with only home adjustment, on a street engine.
Ignition timing and vacuum advance- both fully adjustable on OEM distributors, will usually need to be adjusted for optimum with mechanical secondary carbs. Another reason to dyno tune when changing carbs. Note- high dollar aftermarket distributors often use Ford parts in their internals.
The Ford electronic distributors normally used as cheap drop-ins on Clevelands come from the Ford 429-460 truck engines but the truck advance curve inside can be reset using Ford parts. There are at least a dozen stock advance curves possible. Once again, dyno- tuning will sort this out, too.
Finally, on ANY modified engine, buy yourself a FIRE EXTINGUISHER of a type for gasoline fires! Engine fires are most common on engines with non-stock fuel systems- which with Panteras, is most of them! Trust me- you WILL need it at some point and good intentions 'later' will not save your car. A 2-1/2 lb no-residue extinguisher on a metal bracket will last forever and can be refilled.