According to the HPU tech you may need engien mounts depending on clutch bell and spur gear size.Not sure if your car was a kit or not but for sure reason HPI put 2 different sets of gears in the these.The only 2 ways to know what gears you have in the differentials is to
1 take apart the differential and count the teeth on the gears and divide them.
2 the deciding factor what what car got was by if your car was bought as a kit or rtr.

The kit has a diff ratio of 2.6xx and the others have a ratio of 2.923 or 38/13 teeth