One thing is brake drums turned oversize is they are paired with oversize shoes. The lining is thicker to compenstate.
The other thing is it will pay to have the shoes radius done to match the drum to aid bed in. This can only be done by a reputable brake speciaist on a special machine.
I used your figures and used the Summit calculator and it said use a 37 tooth.
But you are right to slow down the speedo reading based on your driving data you would in theory use a larger tooth count.
I would just try a 40 or 41 tooth and see what happens.
I would get a helper and test both the front and rear operation while up on jack stands.
You may find only the rears are working for example.
The brake drums did not look great in your pic. Have a good inspection of them.
You can get a factory service manual for free at mymopar.
As Katfish said green spring wrong and the silver spring in your pic should not be there at all.
Also note there is a primary and secondary springs at the top.
With the engine off the pedal should be firm and move to around the same freeplay distance each time you apply the brakes.
Engine on the booster will make the pedal "softer" and move somewhat further.
Sounds a bit like trapped air - and readjust the rear shoes.
All wheels front and rear should...
A two speed switch will have two definite clicks or positions from off - low and high.
The 1970 - 71 three speed switch has low as a click then it is variable speed. It feels kind of like the instrumant panel dimmer.
It should be fairly easy to work out what you have, or pull out the switch...
Just my personal opinion - I avoid jacking any car up in a way that twists the chassis or body.
I always jack my car up either under the K member or the rear axle housing.
Does anyone know why GY3 Citron Yella/Curious Yellow was only available for the one year 1971?
Opinion, hearsay or Chrysler factoid welcomed.
Thanks Steve
I used a pair of gearbox clip pliers to expand the self cancel clip - and gently tapped it down with a pin punch.
Helps to have 2 people.
It may pay to bend that prong on the copper ring so it goes up the hole. The wire may end up to short otherwise.