After having another measure/ trying to screw a spark plug thread into it I'm pretty sure it's M14x1.5 thread.

I'm thinking I'll make something or get my machinist mate to make something up. It's currently got a brass reducing bush shoved (loose fit in the thread) in the hole but the cable isn't aligned properly so it's sticky.

At the moment I'm thinking something like you've described in a plate with some adjustment to line it up properly.

I had the same issue, unsure of thread size though, give MADDAT a call.
I ended up making (had it machined) a male threaded hollow sleeve that went through the firewall and had a flat nut machined for the inside of the firewall. Hopefully that makes sense.

Do you guys know what the thread is? My car is also missing the thread, I think it's M14x1.25 but it's hard to measure with the tools I have.

thanks, I found a pic but couldn't make out what it was.

yeah, normally a boss on the firewall which has thread protruding into the engine bay, then the throttle cable has a nut that screws onto that thread to lock it in position, you may need to weld one back on to keep it neat

Brought a maddat throttle cable,

however...the previous owner hacked up the firewall and I'm sure
there is something missing. There is just a rough dodgy hole where the cable goes.

Any help or pics would be appreciated!