[JM 35 Sedan]

It sounds like you may have that 17 tooth gear in the wrong turtle housing. What number(s) are cast in on the outside of the turtle housing you are using?
Those nylon gears sold by Brattons are advertised for model A thru '48 Ford and should be direct replacements for the original Ford aluminum alloy gears.

Edit: Also, I just recently installed 3.54 gears in my 35 fordor sedan. I did have an original #17 turtle housing with the original 17 tooth aluminum alloy gear meshing with a '35-'36 inner worm gear on the driveshaft. The worm gear I have should be the same as the one in your '40. This gear set and the 17 tooth turtle gear gives me an accurate mph reading on my original speedometer.