I run the 416/417 Spec-R Gear Diff in my 415.

This has 37T so i also installed a 37T oneway pulley in front.
If you run the original 16T center pulley i think it will fit also with the original belts.

I run the 18T center pulleys from the 416 as well, so i have to change the front belt to the 169T HB cyclone front belt, wich have 1T more, and the xray 183mm/61T belt in the rear wich is 2T longer as the kit belt.

For the gear diff you will also need 10x 0,3mm shims. I use 5 pics on each side, because the 415 rear bulkheads are wider than the 416/417.

I will post pics later.

Greetings Blueman