For those who have been anxiously waiting for further information on this problem…
We pulled a halfshaft from a 1980 parts car and then one from the 1978 rally car, which you may remember is fitted with a 1987 diff.
The problem was soon obvious. The rally car halfshaft had the bearing fitted the wrong way around. I.e the taper faced out, not in. This meant that the bearing effectively sat inboard by about 2.5mm, pushing the axle flange out by the same amount.
Does anybody know why the bearings might be mounted backwards, or am I blaming an idiot in a machine shop? I’ll talk to my mechanic tomorrow to see if he has any idea. Then I expect I’ll get new bearings and put them on the correct way.
FYI, I measured the shaft as per Greg’s measurements. The 1980 shaft is uniformly wide at 33mm whereas the 1987 shaft agrees with Greg and steps up to 36mm. So at some stage, Volvo strengthened the shafts.