Martin, I tested those 901 flanges in my 915 transmission in my VW chassis using stock VW type1 IRS trailing arms, stock VW type1 drive shafts and stock VW type1 CVs. Everything mated up nicely.
I have since upgraded to the 930 sized flanges which I had redrilled to suit the stock 944/VW type2 CVs, stock 944 turbo driveshafts with early 944 turbo aluminium trailing arms.
I tried to keep everything to standard off-the-shelf parts where possible but I couldn't find a pair of output flanges to fit the 915 transmission that used the 944/type2 CV bolt pattern so I had to have them modified instead.#
just found this pic of the above setup:
(note driveshafts are angled because the transmission was moved forward a bit)