Unfortunately for us Volvo never made an M41/4.10 combination but they did do at least 3 different speedo gears at the gearbox end. I think I've read of black, green and white gears with a different number of teeth. Unfortunately I've never seen a definitive chart of what you use with each combination of gearbox/OD and axle. Someone may already have it written down and there maybe one that can get closer to being correct. Sounds like another project for Ron Kwas?
It's possible to get a small gearbox that fits inline with the speedo cable. They can be custom made to correct the error.
http://www.speedometershop.com/ratio.html
On another site talking about fitting Datsun gearboxes into MG TD's, the price of one of these corrector gizmos is $60. Cheap when compared to a speeding ticket!