British Leyland actually produced the MGB GT V8 for a few years. Not many were built (around 2500) and none were officially import to the US (odd, since the US was the largest export market for MG).
Later, the Rover Group relaunched the MGB body with the MG RV8, a V8-powered roadster, in the early 1990s. Only a handful (not sure of the number) were produced as Rover readied the market to accept the MGF.
All of the V8s used in production in MGs were Buick/Rover-based units. The early MGB GT V8s used the 215cid engine while the RV8 used the 3.9L version found in Land Rovers.
From what I've been told, modifications are minimal to add the V8 engine to the B chassis.
I have not driven any, so I can't tell you how the perform or handle. I do know of a few places that specialize in MGs, some even specialize in V8-powered MGs.