Morris Minor Owners Club

Might not be economic or even feasible but as 3.9 diffs become steadily rarer and/or worn out (and in some cases recently, badly made!), can carefully chosen aftermarket or custom 15" wheels on the rear we used to gear up the transmission on a 4.2 mildly tuned-up MM? Obs. welcome...

Yes they can but you also need to take into account the profile of the tyre i.e. the overall diameter.
You could in some cases have a 15" alloy wheel with a low profile tyre that has a overall diameter which is smaller than the 14" wheel with a 5.20x 14 tyre.
You need to choose carefully.

So possible...but like most things, needs a proper looking into. thanks

in case anyone is interested john brown does 15" and 14" minilites but they're over a hundred each. That's a lot. A 15 on the rear and a 14 on the front might look work quite well? A least for posing purposes
johnbrownwheels.com/wheel-finder/morris/minor1000/208/109

Otherwise I think it might be a case of using a pcd adaptor to go to a more widely used pcd like 100mm or 108mm or 114.3mm. The advantage there is the possibility of accomodating a bit more offset in the wheel, but that is likely to put some unusual strains on the steering/suspension.

Tyre sizes are confusing so I'm using a tyre size calculator at gtsparkplugs.com/TireCalculator.html

Including suitable tyres, costs are already heading in the region of the £500 needed to get 3.9 crown wheel & pinion...but could I fit it? I doubt it very much. So wheel option is still alive at the moment.

JLH minors does minilites style wheels at 4pcd, 5.5" wide, 14" & 15" dia at around £90 inc vat.

To get the gearing up I want using wheel + tyre I'd have to use something like 185/80R15 at the rear which can be got, but I don't think is feasible unless I change ride height of the car and perhaps even bodywork (arches). But interesting, I learnt all about wheels and tyres