All sorted!
So, this was the first time I'd tried to use the column switch since the car was put back together. The horn contact was touching and stuck on.
The first thing I thought when I popped the hood was horn, but the sound was so bad that I decided it must be something worse. Turns out horns that are touching the body of the car sound terrible!
I also had a short in the wiring for the front lights. All sorted now so I can finally drive her with lights and indicators... sort of.
One more problem, she's a 67 so has a short steering shaft. I now have the 67 steering wheel and column switch. But I still can't get the wheel on properly!
Is there some adjustment on the inner shaft, has it possibly slipped down (or the outer shaft slipped up)?
I'm going to pull the whole assembly out and try and figure out what's going on when I get a chance!