Here is the short version.:
Stick with a slushbox in the Dmax. If the '03 has a ZF-6 which needs rebuilding, pray that the input shaft is still good and contact Midwest Transmissions. If the input shaft is shot, you are out of options.
GM version is basically now an absolute relic. Go see an archeologist to try and find one at a dig. Am hearing that the ancient Egyptians might have some in their undiscovered Pharo burial sites. I even had a chat with ZF in the homeland (correct, Germany) and you know the GM variant ZF-6 is a dead item when the manufacturer recommends to look for another automatic versus the manual.
Ford versions are still available via reman. Shoot, I can even convert my OBS ZF-5 to a ZF-6 and just might do that.
Here is a slightly longer version:
Neither will cross-over and bolt to the other (ie: Ford variant will not bolt to a GM block). I did look into whether it was possible to separate the front half (integrated bellhousing section) of a Ford variant and put a GM on the back half with the Ford gears, but the limiting factor was the input shaft. To get a GM input shaft, see the comments about employing an archeologist. Or maybe a physic reader will have better luck.
Gear ratios are the same.
GM used different gears (IIRC helix cut) which reportedly made them more quiet than the standard square gears in Ford's variant. I have ridden in both and neither seemed louder than the other. Downside to the helix gears was that they would not allow the owners to turn up the power as much before they blew. Naturally, these owners blamed the transmission for not handling power way above its design limits. Morons...
GM spec'd synthetic lube in order to avoid an external cooler and circulating pump where Ford spec'd standard ATF (non-synthetic) which required an external cooler and circulating pump. Am not clear whether running synthetic in the Ford variant could eliminate the cooler and pump.
