This afternoon I cut the new hole in the firewall for the mixture control, plus made up some body washers from galvanised steel and used them to plug the holes that are in the wrong spot.
I searched the local hardware store, and auto spares store, for stainless bosy washers, but couldn't find anything suitable. In the end I bought a galvanised steel gutter bracket and used my hole saw to cut four washers, two for each hole (front and back). On the washers placed on the inside of the firewall I fitted a nutplate, so I could then clamp them across the opening, with heaps of sealant as well.
Making up a set of body washers
The redundant holes are plugged
The two eyeball fittings for throttle and mixture control cables are now relocated
