Well, I took the Ranger into work today. Yesterday evening the reverse lockout was "persuaded" out of the old gearbox by drilling a hole in the front of the casing & drifting it out with a punch. I drifted it into the new one & it all worked properly.
That's the good news.
The bad news is it still won't go into fifth so it'll probably have to come back off the car. Oh well such is life at the moment, one step forward, one back.