Bravo 73 even though the MGB is still full of oil, with a shaft failure it doesn't lube the bits which need to be lubed so therefore using the Emlube will at least provide cooling of those critical items in the box so it should be of some help.
And as far as I am aware even though the shaft broke and therefore dropped down a little... it didn't trash the inside of the gearbox...
I stand to be corrected on that one though cause I haven't seen any of the gearboxes myself
RP