Changing the rear hubs is easy. Back off the brakes, remove the drum, spin the 1 5/16" nut off the axle, pull on the hub and give it a crack with your brass/lead hammer. It will pull right off.
Changing the front hubs is a lot harder because you have to change the bearing and then shim them CORRECTLY! I've got a YouTube video about that.
Use anti-seize on the hubs when you mount the new wheels.
I've seen the original 14", and 15" and even 16" used on the MGBs. As the rim gets larger you have to drop the tire profile. By 16" and low profile tires the B is hard to steer and the ride is rough. Looks cool, though.
Use 185/70 tires (or 175/70 if you can find them). Try Universal Tire in Hershey PA or Coker Tire in Chattanooga TN. Vredestein has been the go-to tire recently. Have a motorcycle shop fit the tubes, mount the tires, and balance the wheels.
Hope some of this helps!
Call if you have questions.
SAFETY FAST!
John H Twist