74 or 75, is there a difference?

Mick, the only difference between the slant six wiring and V-8, for '74-'76, is the last @ 1-1/2 foot of wiring.
There is a black connector that you just unplug and replace with a V-8 end, so there is no need to remove all of the wiring from the engine compartment.

I have a V-8 end wiring that will correct this.

Mark.