I think the general rule is to use drive shafts to suit the arms you use. I.e. if you use steel arms, use driveshafts from a car that had steel arms. I've got the *narrower* aluminium trailing arms from late 944 (I didn't know about the alu arms that are the same width as the steel arms BTW) and sourced drive shafts from a similar spec car and they bolt right up to the 915 box and arms. Mind you, I have 930-sized output flanges which I had redrilled to suit the 944 CV joints. See my website for pics of the parts I used:
www.veewrx.com