Morris Minor Owners Club

What is the best way to do this? I note that Moss sell either whole differential assemblies or the 3.9 Crown Wheel & Pinions (would they be compatible with a standard diff?)

First question might be whether you have the engineering facilities to change differential parts.

If the answer is no, buy the ready set up complete assembly.

Thanks. Probably not if it requires any specialist tools and vice etc. Everything else can be done without special tools right?

The crown wheel and pinion can be replaced without the specialist tools but you do need the skills to do the job. It is not a job for the un-skilled.
I would buy the ready to fit and go unit but check with Moss first to see if they need an exchange unit or if an exchange unit is not available from yourself what is the financial penalty..
The Minor 4.55 or 4.22 diff has a 'ATA' diff carrier the 3.9 diff has a 'BTA' diff carrier. The parts are not interchangeable.

It looks very expensive and seems there would indeed be a surcharge. Could be over 0.5p a mile even if it lives to do 100,000 miles. :O

I have asked the restorer to just replace the oil seal for now (the reason I considered changing diff), as I'd probably struggle to remove and re-torque the 190NM bolt myself with a jacked up car.

If it requires dexterity, I'm frankly never going to any good at it. I guess I'll see how many miles I can get out of the current diff and, when it's really noisy, bite the bullet and shell out if I still want to do lots of motorway driving.