The speedo may not be working because the speedo gear housing is wrong for the 3.73 gear. There were at least two "housings" used, one for the 3.08 and lower numerical gears, and one for the higher number 3.42, etc gears. You may have the 3.08 housing with the wrong plastic gear in it so it is not meshing properly.
I am in the process of converting from 3.42s to 3.08s and had to buy the 3.08 housing to replace the 3.42 collar i had previously installed.
https://www.summitracing.com/parts/tci-880022