I finally got the alternator bracket and adjustment sorted out on the M42.
The problem was that the Mocal take-off was blocked by the belt tensioner.

So I cut the tensioner housing out, gave my files a bit of a a run over, machined two 28mm spacers with an 9.9mm dia bore, two M10 rose joints (one L/H and one R/H thread) and an M10 turnbuckle linkage.

I have a few 6PK belts on order, it looks like a Gates 6PK938 should work.