I have had a 3.4L conversion done in my 99MY and while 3.6 and 3.8 conversions can be done, imo, unless its a track car, the additional hp as well as the additional cost involved probably isnt worth it.
Some clarifying points:
- Part of the reason a 3.4L (e.g. '99 996) conversion is easier (and less expensive) other than the price of the engine is because the 97-99MY is non-eGas so no rigging is involved to make it work. 3.6/3.8 engine are eGas and have to be rigged which adds to cost.
- The old transmission can be used on 3.4 - 3.8 conversions, if it's a manual then a stronger sport clutch is used to handle the additional power and i was told a tip should handle it fine as well.
- Some parts were used as donor parts off of my 2.5, such as fuel rails (boxster and 996 are opposite of each other), old flywheel (996 flywheel was about 5-10mm thicker and would scrub the bellhousing if used), a few other small pieces i do not recall.
- My old ECU was used although it had to be sent out to be reprogrammed for the proper mapping of the 3.4, otherwise you end-up with a 3.4 that has approximately the same power as the old engine yet backfires like crazy under hard acceleration from incorrect oxygen/fuel mix. I am not sure whether the old ECU can be used regarding a 3.6/3.8 swap.
- New headers/cats are definitely required for the engine although you can get away with using your old muffler (that is what i am doing presently)
- Stock brakes are fine for daily street use. I'd consider upgrading if it were a dedicated track car.
- Downshifting in some instances becomes an option as opposed to a have-to...
hope this helps some.