Id just upgrade to a w-56 for the $ compared to the price of an R-series. And if you ever have any problems with the W you can just put a R in it later. The only real difference is the R- series is 1/2" longer (I think) and its got a 23 spline output. Which you can easily set up for a decent price.
EDIT: Of course the R-series are WAY stronger and have a lower first gear too but I meant the only real differences in the overall dimensions and spline count. Not trying to confuse anyone.
