The MGB engine I'm building now has been in a MGA for like 20 years, it's already been rebuilt once when originally installed. Now I can't tell you much about the fit of the MGB engine in the MGA, other than it must not be that big of a deal, lots of folks have done and are doing this. The engine I'm rebuilding for the MGA is a 18GB 5 main with the MGA backing plate and a pre-1968 MGB flywheel, no tach on the back of the cam, so I'm assuming they have switch tachs.