New 1/8th BL project, need help.....

I agree, the hyper 8 isn't as nice as the X-Ray. Unfortunately, I don't have an X-Ray readily available to make a comparison; however, if the chief difference is that the diff cup is smaller in diameter, that may not be too big a deal. What's important, I think, is the total length of the space between the diff mounts. You just need to find a diff that will fit between the mounts. I know the losi/sportwerks diff cup will work with the Kyosho spur with a little massaging. We also know that the hotbodies diff will work with the spur, but is a bit longer than the sportwerks diff cup.