Have been told that a 5 speed manual Sierra gearbox will slot straight into my Mk 5 Cortina is this correct?

A type 9 box from a sierra/capri/granada will fit up fairly well, im not 100% sure but there are some small mods needed to make it fit (i think the type 9 is longer than the old 4 speed) - you can also use a MT-75 but it will require tunnel mods and definitly a modified tailshaft (but they are stronger + newer) If your interested in a MT-75 i have a recoed one im not using