[jsceash]

I posted this on another post a month ago.
There are some important questions, What year car? What additional parts are you willing to swap out? Anything other than a porsche engine will cost you in engineering or trial and error replacement. 2.5 and 2.7 replacement with bigger engines will take some transmission clutch and axle mods. (I have experience with this). There are three versions of ECU at least, none of which are upward or downward compatible. All require harness replacements or mods, and ECU modification.

If I were to start over again. I'd get 3.4 Engine, transmission, ECU, Imobilizer, harness, Axles, sub-frames and axle carriers from a 987.1. Send out the motor to be sleeved to a 3.8L, with Carillo rods, JE Pistons, rod pins, and rings. Probably $15k to $25K. With the right tune I'd estimate at the wheels dino of 400HP +. The parts would hold up to the abuse and be reliable. They would be direct bolt in with no mounting issues or modifications.

No matter what you decide if you think you can run 300 to 400 hp out of a 1999 Boxster transmission be prepared to replace a lot of transmissions and clutches and drive shafts or drive shaft bolts. The 99 to 2003 use a 8mm bolt on the axles which will not stand up to 300 HP (Truss Me). If lucky you shear the bolt if not you tear the threads out of the axle hubs. I exploded 2, 2003-04 transmissions 3 clutches 3 flywheels. 1999-2002 axles use smaller shafts than the 2003-06, and the late series 2004 and up transmissions and axle use a 10mm bolt. For 325-330 hp I run a 2005-06 S flywheeel. Spec clutch and pressure plate series 2. 2005-06 5 speed transmission. I believe I'm on my 3rd set of axles currently 2005 axles, speed sensors and 2005 bearing support hubs this is the 3rd year with this setup.

In the end almost every part mentioned was just bolt in. Once you change a motor that is longer or shorter be prepared to spend money to trial and error mount, mount supports, cross braces (they have to be there). moving the transmission over a couple of mm will mean shaft angle changes and transmission mount issues. Then mod an intake transmission adaptor. How to mount a starter with a V motor.

I tried to sell mine twice and the offers were all less than a similar unmodified car.