[A98Coupe]

I did exactly what you are contemplating with my Kirkham. I completely stripped out the 'Kwik Wire' harness and removed the horrible modern fuse box and relays and then made a new harness using the remaining wiring and installing new Lucas DB10 relay and flasher, lucas SRB 111 horn & fan relays, plus new Lucas 7FJ fuse box.
For the wiring diagram, I used a comvination of the Kirkham drawing shown on their website and also the Lucas diagram shown in the Cobra 427 chassis handbook.
Regarding reliability, as long as you buy geniune Lucas parts it's not a problem, and even if there are a few problems, us Brits were brought up on Lucas electrics and are used to sorting out the odd problem - it's all part of the fun!