I'm in the process of this. The shafts are available new for the J type overdrive. You also need the extension housing that is M40 specific, the oil pump cam and retaining spring, and the wide flat spring clip where the spline engages with the overdrive. It's quite hard to get the bits together on my experience. Fully rebuilt box's can be had out of the Netherlands for I think about euro2500 + post if you can get a carrier at the moment.
I like my 140s with m41, they make a big difference. But you have to be determined. It must have taken me 20 year's to get my six m41 box's together without buying new. I'm finding the 164 version even tougher to locate.
Good luck with the searching, it's a fun mission and sometimes things just turn up.