Hi Tom,
Welcome,
Thanks for pm -message, sorry a bit late to reply.
To answer your queries, I looked at some photos with the Takumar 6x7 lenses.
(I use example my -older - photos posted before on PF.)
There are a few RollFilm Back "6x9" and "6x7" dimensional standards.
The Pentax is specified at 55.0 High by 70.0 Wide by my notes.
1)I have the RH_8 Back on the Speed Graphic, which is image size 57.4mm high by 77.7mm wide ( my measurement). With Takumars on the front, I use the Speed's focal plane shutter.
This photo was with the RH_8 on the Speed with the Super-Takumar/6x7 1:3.5/55 (55mm) at f/11 probably.
The Speed was preset for forward tilt by dropping the bed and re-adjusting the tilt and infinity focus, then carried hand held for the photo next day, as I recall.
Box
I think this photo shows that the upper corners of the "6x9" photo are <just> rolling down by the image circle of the Takumar "6x7"lens with camera movements.
2) The metal box camera I made has a machined mount for the P67 bolted to the front of a die cast box, and the RollFilm back is a RH_10 ( 58 mm high by 67.7 mm wide, my measurement) bolted to the rear. The lens is accurately on axis to the back, no movements.
Box
The lens is the Super-Multi_Coated TAKUMAR/6x7 1:2.8/90
This camera can not rival my SLR - RB67 90 mm for close in or low light, but is lighter weight and simpler to use so I prefer it for medium/long distance when scale focus is OK.
Box
Regards, and I wish you well with your 6x9 lens hobby!