Last night I got thinking that maybe the bad starting was just slow cranking. I cleaned the battery terminals and cables and the connection at the starter/generator. None of that made any difference. I measured the voltage drop at the starter/generator and found it around 4 volts! Not good, no wonder it cranked slow. I had some 8ga wire Fred had given me for the '78 240D which I'd never used and a couple crimp on ring terminals so I made up a new cable from the battery to the starter, this one was the winner, the old one was small and had a splice in it to an even smaller wire.
On the left the old wire with the splice, on the right a comparison of the new wire and the old, see how much bigger that new wire is.
I also made up a new lead from the battery to ground. Honestly I'm not sure how the old ground worked at all, it'd been attached to a painted surface. Again my new wire is much larger than the old. I also put heat shrink tubing over the new terminals. The existing terminals on the cable already had heat shrink.
With my new cables installed voltage drop at the starter/generator is now less than 1v which is entirely acceptable. I'm still going to replace the wire from the switch to the starter/generator. I would have already but I ran out of ring terminals and it was too late to go to the store. This is probably excessive but when it is done I'll know that the cables are all good and that I'm getting every bit of starting voltage I possibly can.