Hmm... I've run into late model 351-W distributors that are somewhat taller than 351-Cs for no known reason. With most Ford V8 distributors 1957-1985, the gears and most of the insides interchange, so it's possible what you have is a MSD 351-Windsor distributor with a 351Cleveland gear on the shaft.
The combination will work fine in a Pantera with a 351-C using a Cleveland gear, except for rubbing the wires on some engine screens. The Cleveland gear is noticably larger in OD than a Windsor. Do NOT try running a Windsor gear in a Cleveland as only the very tips of Windsor gear teeth will touch the Cleveland cam gear, and will start up but will ruin both cam and dizzy drive gears in a very few miles!
Note also that some Aussie Clevelands used in Panteras assembled outside of the U.S, had a smaller OD distributor shaft and those blocks also have a smaller bore hole for the oil pump extension driveshaft so the gear interchange will not fit without machine work. Hope this is what happened.