A few considerations:
The cheapest I have
ever heard of it being done was for $5K, and that was with a ton of wheeling and dealing on the parts. The result was a nice little Six with a rather tired 2.0 liter 110 HP motor, tail-shift tranny, four-lug wheels with stock brakes and 195-width tires, and so on.
On the other end, you have the $50K+ car at the following link:
http://www.pelicanparts.com/914/914_photo_gallery/Karl_Johnson/johnson_914.htm
I'm guessing that a "mild" conversion will run you $15K, and that the folks planning on doing 3.0s for this kind of money are going to be a bit surprised when they add up their receipts. You can trim costs by doing more of the work yourself, and costs will increase (potentially a
lot) if you have someone else doing the work.
The 3.0 motors need a special flywheel to fit the 914's transmission. The OD fits the tranny bellhousing, but it has the mounting holes of the later flywheel. The 3.2 and 3.6 require a different special flywheel, which is evidently less common and more expensive. Particularly, if you want to run the DME FI, there are specific bits of the flywheel that need to be there.
The 901 tranny is not up to a whole lot of torque. It'll cope with a stock CIS-equipped 3.0 pretty well (just ask MikeZ), and I know one or two stock-FI'd 3.2s who use 914 gearboxes, but much more than that and you'll start running into trouble. Tony Inae on the 914 Rennlist has broken several 914 gearboxes with his 3.6 DME motor. I'd guess that with a 3.2 you'd have to baby things a bit.
The next upgrade (and it's fine for anything short of a BIG V8 or a hi-po 930 motor) would be a 915 gearbox from a 72-86 911. Mods are required to make it run correctly in a 914. Look into the prices for the ring and pinion flip, the clutch linkage design and fab, and the gear shift linkage design and fab, before you decide to go through with it. And check into how the different designs of shifters feel--the one that I've driven is really awful.
There are other considerations as well, of course. Check out the "914 conversion" section of our website, it has a ton of info on the subject.
http://www.pelicanparts.com/914/914-6_construction_kit/914-6_construction_kit.htm
--DD
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Pelican Parts 914 Tech Support
A few pics of my car:
http://www.pelicanparts.com/gallery/Dave_Darling