In the UK one of the most popular suppliers of belts (harnesses and retractors) is securon
Seat belts, seatbelt and harnesses manufactured by Securon for all vehicles and motor sport
- very helpful technical dept to.
Here are some pics of my Crendon with central seat belt mountings:
DSC00591 - Cobra Club Gallery
- but this is a static 3 point belt, although i am upgrading to retractors at some point this year. The bracket you see is part of a 'tower' that is bolted to the chassis directly above the diff. It enters the cockpit via a slot.
there are also some pics of the central mounted retractors on my old green car (a Pilgrim) - these bolted to a steel bracket embedded in the fibreglass. it was effective, but less aesthetically pleasing.
this also may be useful, pics of the Hawk arrangement, which is very similar to the solution used on AC MKIVs using inertia reel retractors.
Building our AC Cobra 289 LeMan » Installing Seat belt framework
http://www.the289register.com/stuart/Part%2013.pdf
http://www.the289register.com/stuart/Part%2026.pdf
HTH
Kevin