Series ? gearbox

["Paul Tring" <paultring@blueyonder.co.uk>]

Hello
I've got a Series 5 with standard gearbox, I recently managed to get hold of
an overdrive gearbox (advertised as a Series 5).However after dismantling it
is obviously an earlier series i.e reverse on opposite side ,no synchro on 1st
gear. Does anyone know if the mainshaft from the overdrive box can be fitted
to the series 5 box to enable the overdrive unit  to be used, or is my best
bet to try and sell/exchange the gearbox for a later one
Paul