The very early TDs had a smaller flywheel with a 7 1/4 inch clutch. The gearbox associated with that engine has a smaller bell housing and input shaft which will not accommodate the later clutch. Those engines carried an engine number of XPAG/TD nnnn, where nnnn is a number less than about 9800.
From TD engines with engine number XPAG/TD2 nnnnn, the flywheel and clutch were changed to an 8 inch clutch, and the gearbox was changed to accommodate this. All later TD and TF engines, even the XPEG, will fit the later gearbox. The only other difference is, of course, the thermostat housing and radiator connection, which can be easily changed.
I know, as I have done it.