Theoretically you can lift the engine and gearbox on the (centrally located) rocker cover stud on it’s own!…….and certainly when i was lifting the engine there is no doubt that is the case.
I felt happier with a piece of angle iron drilled to fit over both rocket cover studs spreading the load and to prevent any leverage forces increasing the load on the studs. When lifting all the weight appeared to be over the one stud as the chain was pretty well slack over the other.
From what i can find on various forums on the internet the combined weight of engine and gearbox is less than 250kg.
mgb lifting bar 2
mgb lifting bar
MGB tips mods and maintenance 