Reshafting to a Belly Putter

I tried a friend's TM Rossa Belly Putter and the feeling was very good.

I want to reshaft my conventional length Odyssey (White Hot #7)mallet putter to a Belly one.

After researching a bit, I found that there might be more science and fitting to it then just running to a store and grabbing a 40" shaft.

Optimum shaft length. shaft angle, the "ideal" head design (centre shafted vs others), "ideal" head (Mallet vs others), Head weight (400-500g)

Any bros here currently gaming with one now? Did you pick it up off the shelves or did you get it reshafted? If reshafted, where did you go? Any fitting done?

Thanks