The original cars used Lucas (prince of darkness) wiring looms. But modern reproductions use modern wiring harness. You can still get reproductions of the original wiring harness from places like Cobra Restorers, but that may open a can of worms. Still, you can make the firewall look more original using the old components but making them non functional. Here is a list of the components
C5TF-FOMOCO, OEM style external regulator (AMK)
115.003, Lucas SFB105 flasher (Retro Classic)
115.018, Lucas SRB146 relay (Retro Classic)
33117, Lucas DB10 relay box (Retro Classic)
4FJ, Lucas fuse box (Various, i got mine off eBay)
Here is a picture of the firewall of an original for reference
You'll also want to use SW mechanical gauges
Cheers
Bill D