There are any number of off-the-shelf RWD packages that would work well in an MGB. I've seen Toyota, Nissan, Mazda, Duratec, and Zetecs put into these things. The Zetec/T5 is compact, fits well, and weighs almost 200 lbs less than the MGB engine and trans. Personally, I wanted to use a 4.0 Rover V8 that I have at my friend's shop! I think a Duratec would have been a good choice, too.
Just like the Miata, the Zetec has a lot of community and aftermarket support. The Brits routinely use the engine in everything from early MkI and II Escorts, Fiestas, Mondeos, and a host of others. They've done the goundwork for most of these kinds of swaps.
Then when we looked at the stuff we had leftover from other projects just laying around the garage it kind of made sense. We already had a bellhousng and a T5, a Megajolt controller, Mustang radiator, bike carbs, SU's (if we don't like the Keihins), and a lot of other bits. We have access to machines and fabrication tools, so making a bracket, milling a piece of aluminum, or turning a shaft is no big deal.
There's also was the cost. Zetecs are dirt cheap. We paid $250 for our low mileage engine, but have run across them for as little as "free". Not so with the Miata stuff. Good 1.8 engines and 6 speeds are still a little pricey. For the price of just a single 6-spd miata trans, I can buy five T-5's.
As far as easy, this swap is pretty straightforward. The flywheel fits, the trans and bell bolt on. Heck, the only hard part is making a water manifold to run to the radiator from the back of the head. I think there's probably nearly as much fabrication with any combo.
Finally, when you are tire-limited for traction, you can only use so much horsepower anyway. 10 or 12 lbs/hp makes for a pretty lively car. The MG will only weigh about 2000 lbs when we get finished. With 150 RWHP or a bit more, it will be pretty quick.
Anyway, it is what it is, we have it and are committed to making it work well.