71 bus change diff?

I bought a 71 bus a while ago and i picked up a 2110cc type 1 motor with twin 40dells from the US to pop in it.
The gearbox seems a bit undertoothed for the new motor. What are my options to up the ratios.?
Somone suggested changing the diff, can i change it for a different std vw one or can i find an aftermarket one that will do. It has the 1600 box in it.

why not go with a 1800 or 2L g'box?

yep can highly recomend the 1800 a real highway runner