The cells are direct fits in a Compur/Copal/Seiko #0 shutter. In K-O mount they're in a Copal #0 modified to be fired from the body and cocked by the body.

They will not fit in a press (self-cocking) shutter, the inner element of the rear cell will hit the diaphragm. Don't waste your money on a press shutter, get a proper cock-and-shoot shutter.

If you go ahead, you'll need to have the shutter scaled for the lens.

I've done all this with a Koni/Rapid Omega 60/5.6. Wanted a 58, but a 60 turned up at an amazing price.

The 60 just covers 2x3, absolutely won't do for 6x12. I doubt the 58 has more coverage. It doesn't have to and like the 60 it has a relatively small last element.