The original V8 optioned cars used an improved GM 215. I would stick with GM.
Frankly, I think going with either of the two is ridiculous because the chassis simply isn't heavy enough to put all that power to the ground, all it would do is burn up tires. A 200hp MGB would outperform a 500hp one, handle better, etc. unless you're doing a build that uses nothign but the cosmetics of the car
When the MGB was built, it was with the aim of producing a well-balanced, great handling sports car that was competitive with the sports cars of its time, and wasn't too expensive. It wasn't the fastest, nor the most powerful, but it was a decent competitor, and people have raced them with the factory components for many years. Fast forward to today, and we see the Mustangs, Camaros, Challengers, Chargers, WRX's, and a host of other cars with over 300 hp. However, these cars are physically larger, have the capability to put the power to the road with larger tires, and have more modern, maybe more appropriate suspensions and brakes. The MGB chassis, suspension, and braking systems will be overwhelmed with 300+ HP, and will be hard to control on the road or track with that kind of HP. If you just want smoke, go for it, because that's what will become of the rear tires. However, if you want a serious car, go with a Stage 2 or Stage 3 MGB engine, or a more modern 4-cylinder or 6-cylinder engine tuned to around 200HP or less.
It will improve the acceleration of the MGB to the point of exhilaration, but will be much safer, especially around the twisty's.
Scott Stuchell