Be careful with your off-roading as the radiators are not designed to take very much abuse. The inner mounting stub is known to break easily when guys go off track. The inner stub is also known to break just from constant use. I moved my radiator to inside the frunk for that reason. My boxster is dedicated more to rally cross though. Here's my build thread so you can see what I did. http://986forum.com/forums/show-tell-gallery/73877-6-months-live-build-thread.html
The seat and harness set up will not pass a PCA tech inspection. I would encourage you to talk with the organizations you plan on running with regarding their minimum standards/rules. This is a way to ensure your build is in compliance and you do not have any issues once you get to the track.