I have done some extensive work on my '68 912, including lengthening the wheelbase, replacing the 901 with a 915, body mods, interior mods, and V8 conversion.
First, from my research, the 901 can live with up to 300 HP if it's in good shape, but I am not sure of how much torque it can withstand. The 915 is much stronger and regardless of what you may hear, is not terribly difficult to swap in place of the 901.
I tried several approaches to lengenthing the wheelbase, since I added wheel flares which are typically for LWB cars and made the stock rear tires look pushed forward. First I tried swapping in rear control arms from a '70 911S which are longer and became a nightmare, but will work if you are very determined. The main problem was that the track was much wider and the springplate was lining up about 1.5 inches inside the bracket on the control arm. I fabricated spacers and used a second set of springplates to make an offset setup that works well. Due to my own preferences I scrapped this approach.
The second method (which is what I am using now and works well) was to relocate the front mounting location for the control arms rearward about 2 inches. This has been used by the factory on some early race cars and I learned of this from Bruce Anderson's book, Porsche Performance Handbook. This allowed me to use all of my stock components. Once again, the springplate alignment was off, but only by about .5 inch, which was easy to fix (since I already had fabricated special offset springplates, I just used a .5 inch spacer) and this set my wheels in the center of the wheelwell and widened the track by about an inch. It looks great. If you stay with the 901 you can bolt up the stock axles and off you go. If you go with the 915, you may need to change the inner CV joints to fit the 915 stubaxles. I am currently using the 901 stubaxles in the 915 and using my stock axleshafts, but there is a little play and the jury is still out on whether is this a bad thing. Also, depending on the shocks you use, you may need to clearance the upper shock mount since the shock will be angled more to the rear and may hit when the supspension is in full droop (mine did.)
Here's a look at my car. It has flares and whaletale. It started life a 1968 912 sunroof coupe.
I can't share much about the 912 engine, my car had a worked over VW motor in the back when I bought it, and it did have an upright (doghouse) fan. 1.8 liter and about 100 HP, I was getting spanked by Honda Civics. That is unacceptable. Since building a reliable 350 HP Porsche is a 4 to 5 digit $$ proposition, I went with the V8 conversion. It's some work, but I am very happy with it.
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Jay
'68 912 V8
[This message has been edited by JordonJames (edited 06-06-2001).]