Tony
If you are getting your MOT done at a friendly garage, mention that you want to change the gearbox oil but can't find the drain plug. If they are as nice as my local village MOT garage they'll probably point it out to you while the car is on the lift.
There are modern gearboxes made where you have to such the oil out through the filler, but I'd be surprised if one from 1975 was in that bracket. I'm not ruling out the drain plug being above the cross member (mine isn't) but a few spanners to undo the single mount and a trolley jack to raise the gearbox a bit would be one way if that was the case.
