Hi Mathias,
the 912-gearbox is not the best gearbox to recommend in your special case. In the 912 a type 901 gearbox with short ratios was installed. It was no 915. For the type 4 engine that you have I would recommend a 914-gearbox. Their ratios are much longer and they fit perfectly to huge type 4s.
A 915 is not necessary. The 914s stand upto 270 Nm easily and are much cheaper and easier to get than 915. Besides 915 are about 52-55 kg in weight, a 914 is about 42 kg.
If you´ve got more questions, please contact me by mail to
info@bugat5speed.de, or have a look on my site
www.bugat5speed.de
Cheers
Martin Bott