A 915 tranny will cost almost as much to flip the ring gear and install.
I looked real hard at that option, but ended up staying with the 901 tranny (being a cheap so and so).
BTW, renegade hybrids and Rod simpson both offer modified 901 side shifters with higher 3rd, 4th, and 5th gears. oops, just saw that Rich posted the same thing, beat me by this much :-)
I also remember seeing someone trying to sell a 930 short bell housing tranny all set up for a 914 V8 in the recycler
www.recycler.com for around $3500. No idea if it is still in there, but it is worth a look if you are in the market.