Take your hint from NASCAR and many other pro race teams and use TWO MSD boxes, so when- not if- the first stops sparking your plugs, you can switch to the second unit and make it home (or at least back to the pits).
For most Panteras, ANY brand of large cap distributor WILL NOT FIT. A big cap may- possibly- be made to clear the firewall, but the first time you tromp on the gas, the engine package will shift slightly on its motor mounts and the cap will hit the firewall. That will shift the cap and break the plastic rotor. The engine will abruptly stop. Unless you carry spare parts, you're going home on a tow truck- if you have cell phone coverage.
You can alter the motor mounts so the whole powertrain is moved backward at least 1/2" so big caps fit. Otherwise, use a small cap distributor. Early screens seem to have less distributor clearance than late screens with the metal center. Or if you enjoy fiddling, you may utilize a slightly shorter 351-W distributor body with a Cleveland gear and make a little more screen clearance that way. Fact: all Ford Inc. distributors 1957-about '87 were essentially the same and internal parts swap around for tuning. Exception- some SBF dizzys used a smaller shaft which won't fit the Cleveland gear.