The mondeo box will fit the engine, but you'll have some trouble with the gearchange.
the RST box is a Ford BC gearbos and has been used in all Escorts/Orions/Fiestas... form around 80 to around 95. Surely you can find one such gearbox. When you have found one, you can take your limited slip differential and put it in the new gearbox. Much easier this way.