Dale,
I think no one has replied because the answer is not there.
The parts manuals and supplement only list the length and width measurements and not the depth/thickness of the cushions.
After 70 years I do not think there are many original cushions left to measure.
I would make them to whatever your bottom & back are comfortable with. If you have a large frame you may wish to reduce the thickness of the front cushions, or if you wish a more comfortable ride try thicker cushions (or another type of vehicle
)
If they are in proportion to the rest of the cushion dimensions and you are happy with them, go with that.
With foam material and what is available today, there can be some variation to density of the foam and therefore the thickness.
Regards