I have a '41 Plymouth 1/2 ton. I can't verify 100 % that it's the same for 1940 but written information I received years ago from the Plymouth Club tech adviser was that the front sits on 3- 3/4 inch thick blocks plus 1/4 inch thick rubber and the rear sits on just the 1/4" rubber.
I am currently putting everything back together on my truck to get all the gaps and clearances right for the front nose, hood, fenders and running boards before it all comes apart again for the painter and that thickness of blocks is working out good. That puts the flat part of the cab floor where the wood panel sets just off the frame rail a little bit.