It is supposed to be quite simple, based on what I have read and what a friend here in Calgary has done. John Warga is converting his auto GT to a Getrag 5-speed, but the conversion to a factory 4-speed would be similar. If you have the money, the Getrag would be a far more satisfying conversion.
There aren't any "kits" available, but the basic stuff for the clutch is present on all auto GTs. You just need a donor car (and it can be either a GT or a '68 to '72 Kadett) for the pedal set and the clutch cable (the GT is different than the Kadett). Then you need a pilot bearing (inside the crank, the auto doesn't have one), flywheel, pressure plate, clutch disk, bell housing with release fork and bearing, a 4-speed drive shaft, rear cross member and the transmission, and finally the shifter and shift boot (which are different on the GT than other Opels).
If you get a Getrag, it has an integral bell housing, and the auto drive shaft you have is the preferred donor after shortening, and the auto cross member is also required. You might also contemplate doing the S-10 clutch conversion and lightening the flywheel (actually in either case).
There aren't any interior mods required, aside from installing the pedal set (which has the clutch and brake pedal on a combined shaft)
gmeyerle: if your son goes through with the manual conversion email me at [email protected]. i have a good 4 speed, multiple bellhousings and clutch forks, clutch cable, pedal set up for the manual, flywheels, shifters, and a NEW pilot bearing. all you'd need is the pressureplate, disk, throw-out-bearing, and clutch tool, which i got for 125 bucks brand new from a local "campbells". it was manufactured by sachs. i'll let you have it all cheap, hurtin for money with christmas comin up.