The JP Spec kit includes a door-side bracket designed for M3 sedans. It allows to bolt the belt's anchor to the seat, as engineered by BMW.

In this entry, I discuss the M3 V8's seating problem and the Recaro solutions.
I go in-depth on the RECARO JP Spec mounting solution and compare seating position with OEM and Recaro's EuroSpec solution for the M3 V8, 1M and F8x Mx cars
TL;DR: It is the lowest sliding mounts for these cars - and it uses all RECARO original hardware.
Enjoy,
-Matt
Been there, done that.
The Bespoke Recaro Seat Program offers near infinite customization possibilities created from Recaro seats.
Leather, fabrics, threads, stitching branding, electronics' integration and other aspects of the seats can be fully customized to your specifications.