I've been ruminating about this for quite some time. I have a 3.6 pushing a 915, and I'm not happy with the gearing (close ratio for track use with a short top gear). One day it will come out, I'll rebuild it with new gears, and thousands of dollars later I'll have a 915 that shifts decently for a while. I think I could get four gears in a 930 that shifts well and handles more power for twice the cost, but I'm starting to wonder if a similar amount of money could have me driving around with six modern gears instead.
I've read about people putting Subaru transmissions in 914s:
http://www.914world.com/bbs2/index.php?showtopic=110990&view=findpost&p=1368310
914World.com - The fastest growing online 914 community!
I've read about people putting 915s in 914s with an adapter from WEVO:
Transmission > Porsche 915 WEVO to 914 By Patrick Motorsports Porsche & Mid Engine Performance Specialists
Subaru Gears sells a kit for reversing the output of Subaru transmissions for use in various old VWs, as well as
"930" flanges to mount the CVs to. A salesmen told me he's seen one go into a 912 with a Subaru motor, but I haven't seen pictures yet.
If a 915 fits in both a 914 and a 911, and a Subaru transmission fits in a 914, what are the largest barriers for putting a Subaru transmission in a 911 in place of a 915?