what should the thickness be on a new thrust washer, the brass ones
New bronze thrust washers are about .090.
New ones from Bobs or Langs are about .205" thick.
You usually need to "surface" them down a bit.
The fit up of the ring and pinion gear set is determined by the bronze thrust washer thickness. The second bronze thrust washer thickness will set the clearance of the rear axle internal assembly within the outer axle clamshell.
You need the Model T Shop Manual reprint and the MTFCA rear axle manual. They will cost you about $30 or so and will be well worth it.
An orbital sander with a fairly coarse grit paper will do a good job of sanding down the bronze washers to the correct thickness. Stop every so often and check the fit.
I make them different thicknesses. That way they can be juggled from side to side which can be a help when setting the ring and pinion gear clearance.
Allan from down under.