If you are running the stock frame you just need the measurements for the cab floor off the frame rail.
It reads .78 between the top of the frame rail and the cab floor.
I've been thinking about doing the designing for a while and this is what I came up with,
Fab the bracket so that it allows for the rubber or poly cushion and leaves space for shims that you make.
The revised (if you can call it that uses the two bolt holes that you would use to attach the stock shackle mount to the cab floor with a rubber/ poly mount . I used 55/57 Chevy Motor mount biscuits for the body mounts when I put my 51 Mercury body on the 75 Monte Carlo frame and that worked well. They are easy to find, and not expensive.
A guy would have to do a bit of trimming to get two of them to fit side by side with that bolt pattern though.
It should be pretty simple to lay out the holes for the bracket to the cab. One could make the bracket a bolt on unit if they wanted to. I'd rather leave a bit of space and have to add shims to get it to the correct height rather than find out that I had the bracket too high on the frame and needed to make changes.