1967 Fastback Wiring Harness

Just got my car out of paint shop a few days ago. Interior is all removed right now and want to replace all wiring from front headlights to back before installing new interior.

Been seeing a few wiring update kits out there like C510 and American Auto Wire. They look identical in the pictures. C510 says theirs require some modification to the firewall and comes with template so I am thinking it may require enlarging the original hole??
I am trying to use as many factory replacement parts to keep as original in those aspects as possible and really prefer not to cut on the firewall.
This is not an all numbers matching car as it came originally with 273 Magnum/4spd, if I interpret the VIN and fender tag correctly but had 340 in it when I bought it. Everything else matches as far as I know. Also went back with original MM1 Turbine Bronze Metallic paint. Fenders, hood, grill, bumpers, front valance, doors, hood hinges and trunk lid were removed and painted separately.

Anyone out there know if anyone is making a factory equivalent replacement wiring system that doesn't require cutting/altering the original firewall hole?
Will post some before/after pics on the projects thread if anyone interested?
Thanks. Appreciate any help.