Contact these guys, and they can fix you up with the correct components:
Discovery Parts
There are a lot of combinations of seats, harnesses, and adapters, and these guys know what works. Part of the confusion comes in the way Brey-Krause (B-K) packages their parts, in left and right side groupings, and multiple adapter options and belt options.
In general, you are going to have to pick a race seat and decide if you are using a HANS device first, and then go from there. No sanctioning bodies that I know of will let you use a harness with the stock seat for some reason.
Good luck.