I have just done some measuring.
Distance from the film plane to the front of the body: 45mm.
Distance from the back of the lens (front of lens board) to front of body (at infinity): 25.5mm.
Total distance: 70.5mm.
Schneider data sheet figure for film plane to back of lens: 70.5mm !!!
I was originally going to mount the helical mount on a plate and mount the plate on the body. Now I intend to mount it directly to the body but recess it by 3mm. This will hide the hyperfocal marks on the barrel (which will be wrong) and move the lens too close to the film. To counter this I will have a shim area to bring the film plane back to the correct place which I will find by using ground glass rather than an absolute measurement.
As the film gate and spool holders come out as one unit which locates on pins, it just needs a suitably sized spacer to butt up against to ensure the film distance is always correct.
Next week I will bite the bullet and start cutting the oak to make the body. I only have just enough for the body so I expect I will make a grey PVC body first or possibly one from 3/8" MDF.