Mick,
The Mk4 & 5 Douglases had bronze liners in the crankcase to carry the rear main bearing. These liners were in the form of a headed bronze bush. Now, is the hole you have found part hidden by the flange of this bush? If so, then there should be a shallow headed pin in the hole and a corresponding scallop out of the back of the flange to accommodate the head - this was done to prevent the bush from turning. Maybe the bush has been replaced at some time and doesn't have the scallop (which should have a depth about half the thickness of the flange). Normally, the bush stands proud of the crankcase by a few thou, so should get clamped into place by the clutch release assembly, so may not be reliant on the pin to stop it moving. If you don't want to disturb the liner, you could tap a good fitting pin into the hole from the inside and then peen the hole to prevent it from working it's way back out.
If there is any sign of looseness in the camshaft plugs, remove them and clean the area, then Loctite them in. Then, to be double sure, centre punch around the plugs to make sure they cant move. Plugs have been known to get dislodged and then get caught up in the clutch release. Be careful when rebuilding the motor - do not try to tap the timing gears onto the camshafts as this can force the cams back until they push the plugs out. It is easier to fit the gears prior to assembling the crankcase halves as there is only just enough room in the timing chest to get the gears on the camshafts.
Good Luck with the rebuild.
Regards,
Eddie.