Detonator, My intention is not to rain on your parade, but be aware that your beautiful #1442 seems to have what's known by RBT Transmissions as a 'dash-1.5-series' ZF transaxle. This was an intermediate-stage ZF that uses the upside-down differential case needed for all Panteras, but due to parts shortages at ZF during the early days of Pantera production, an unknown number of early cars (mostly pushbuttons but obviously not all) got a Mangusta transmission bolted onto a special 4-bolt bellhousing (later used 6 bolts), and some also got the Mangusta ring & pinion gearsets. There are a myriad of parts inside that do not interchange with the 5DS-25/2 ZFs found in later cars, including the entire clutch, bellhousing and transmission mainshaft. Kirk Evans may have more specific info.
There's nothing really wrong with those transaxles- the gears were used in GT-40s, except that -1- if used, the gearsets are lower so acceleration is absurdly high (Goose ring/pinion ratio is 4.50:1 vs the Panteras 4.22:1 and overdrive 5th gear is 0.846 vs .705). This gave a top end at hydraulic-lifter-redline 5900 rpms of about 130 mph vs the later gearsets giving nearly 160 mph. This was also the source for some early magazine road-test disparities between various Panteras tested by "experts".
-2- Those four long tie-bolts & spacers between the bellhousing and ZF side-plates are notorious for loosening and eventually cracking the side plates, leading to a major lube-loss. Check the torques frequently! Some simply leave the bolts out.
-3- Spare parts for this combination from RBT are VERY scarce if something major goes wrong. Hundreds of such ZFs were scraped by Ford dealers at no charge to owners back in the '70s and other owners have purchased a later series spare to drive on, so what you have there is a virtual collector's item. I've seen maybe six others running in 33 years. Have your photos handy if you need to order parts for the ZF or clutch, and shift that big motor carefully....