Peter, if the M1 ZF's input shaft is, indeed, longer than a -2's, the extra length would not be on the engine end of the shaft. The two are not interchangeable although I was not aware that length was the issue. The pilot shaft is smaller than a -2's but the rest of the input shaft is more robust. I can most certainly confirm that an M1 ZF can be installed in a Pantera without issue.
I purchased my M1 ZF in 1999 from Roland J. in Germany. If I recall, he had just bought 20 or 30 of them from BMW Motorsport. They were brand new and in BMW shipping crates. At the time, my -2 was with Lloyd Butfoy, about to be rebuilt. We (Lloyd and I) determined it would be cheaper to buy the M1 ZF and convert ("flip") it for use in a Pantera than it would be to have my old -2 rebuilt. That included a taller R&P. The cost of an M1 ZF back then was less than 9,000 DM. Roland sold every one of them in a matter of hours!