John, I have no idea if thee MGB wheel nuts will fit.
The Plus Eight front hubs originated from the Rover P5 and P6 and the thread size from memory is 9/16 UNF.
An equally critical aspect of the wheel nut to the thread is the convex face that located into the steel wheel insert. This are rounded and seemingly identical nuts have different curvatures.
All I can do is suggest that you take one of your nuts to an autojumble at a classic car show and find identical ones.
Finally, stainless wheel nuts look pretty but be aware that the material does not have the same elasticity and stretch properties. Having said this, I have never heard of wheels falling off due to stainless nuts coming loose. Personally, I would definately not use them if the car is driven in competitive events or track days.
