BBI seat base and side mounts, I bought a bolt and spacer kit from Tillett (for something harrowing like £8), and a 964RS seat-belt receiver bracket.
The brackets bolt spacing isn't the same as the BBI base so I have temporarily modified it, but I'm getting some inserts machined and will TIG them into the BBI bracket next week (I wanted the seat in the car for Precision tomorrow).
The bottom of the seat is actually between the rails, and lower than the upper surface of said rails - very low, as you can see in the comparison to the stock hard-back sports seat.
This means that I can see all of the clocks, for the first time, which was one of my goals.
The other goal was to see what people were talking about when they said that buckets made a difference to the feel of the car - and all I can say is wow.
Being held in place as the car turns is a totally different experience to sliding sideways in the seat (even heavily bolstered sports seats). It's not clear to me yet which was the most transformational modification - the Ohlins or the seat.
Both together and you've got an incredible boost to the driving experience.
I took it out for a spin last night and it's the first time I've seen the TC light flashing as I accelerated out of corners, which may of course mean that I'll be finding out just how strong these seats are in an impact fairly shortly, but hopefully not.
I strongly recommend this as an upgrade, so far (!) the only downside is the lack of heating, but I'll get that rectified at the end of summer.