MG actually made a B (MGBGT V8) so it's not a big deal and the motors were used in everything as I recall. So while not difficult to do... I wouldn't. The B is a turd, a fun little turd but a turd none the less. No point in giving it big power it just isn't a good idea.
I buried thousands upon thousands and untold hours into a B back in the 80's-90's. Learned a ton about cars including one very important lesson...stay away from cheap British sports cars
In all seriousness though that chassis just plain sucks.