I agree with radar. 4mill cubs are pretty impressive, but like everyone I've known you always wish you would have went bigger, so if your planning on building a 10mill, go with it, or your gonna be pissed at yourself if you went smaller and arent satisfied. lol
There's a 10mil crank on ebay right now, see how cheap you can get that and your on your way.
I'll tell you this, they are a blast. There has to be around 25 10mil cubs where I ride. There a great motor, and very hard to beat.