For the conversion crowd, upgrading the MGB's salisbury axle is pretty common. The Ford 8" is not a bad choice. It can hande 300 hp without modification and there are quite a few choices for gear ratios. One drawback are the drum brakes. Upgrading to disc brakes doesn't have to cost a fortune. It can be done for as little as $130 using common parts and just a bit of welding.
Starting off, Speedway motors sells weld-on caliper brackets for $8.00 each. Calipers - you can use the fronts from a late 80s chevy S10 or simiar vintage monte carlo. These calipers can be had for about $30 for a pair. New caliper pins are about 4.50 each. You'll need 4. The down side to using front calipers is no parking brake. Another option is calipers from a late 80s cadillac seville - the rear calipers will have a parking brake mechanism. For rotors, you can use a set of rears from an 87 lincoln lsc - $40.00 for the pair (which will match the ford's 5 x 4.5" bolt pattern - Mid 90s ford explorer rear rotors will also work). Brake pads can be had for about $25 for a set. If you have a different bolt pattern you may have to do a litte research to identify rotors with the proper bolt pattern. My approach will give you 10" rear discs. Should be plenty of stopping power for an MGB.
Now for the dirty stuff. Removing the drums is pretty straightforward. Remove the road wheels. To get the drums off you may need to release some tension. On the backside of the backing plate there is an oval hole - you can use a small standard screwdriver or a brake adjusting tool to back off tension on the star wheel inside the drum. Once released, the drum should slide off - perhaps with a bit of pursuasion. With the drum off you can reach the four bolts that hold the backing plate on (and retain the axles. Remove the bolts. To get the axle out, you may need to use a slide hammer - attach to the axle flange - give it a few hits and everything should come free. Repeat on the other side.
The only trick is that you will need to replace the thickness of the drum backing plate. You can cut out the center of the old backing plate and use that as a retainer - or fab up a retainer from some flat steel stock (use a 3" hole saw for the axle and drill 4 holes to match the axle housing flange) My shortened axle housing and moser axles needed a 3/8" spacer so I used some flat stock - local steel supplier just gave me some scaps.
Reassemble your axle with your new retainer. Fit the rotors and bolt them up (without road wheels so the rotor is flush to the axle flange). Assemble the calipers, the Speedway brackets, and pads making sure the bleeder valves are pointing up (calipers at the rear of the axle). Position the assembly over the rotor and align so the pads line up with the rotor. It may help to have a friend hold things...tack the bracket to the axle. Recheck your pad alignment - if it looks good weld the bracket in place. The welding process can pull things out of alignment a bit - welding both sides of the bracket shoud pull things back into shape - if not - a few taps with a block of wood and a bfh will get things back in shape - repeat on the opposite side and you now have rear disc brakes on your ford 8" rearend. There are bolt on kits available as well - these are usually in the $600 range compared to $129 to make your own. Using the seville calipers will add about $70 to the total - but you get the parking brake.