Hi Philip, Billy Bellinger at JB Engineering hould be able to assist you although it may be easier to obtain a complete crown-wheel and pinion differential assembly and get it fitted locally.
Beware, to remove the differential assembly from the axle, one needs to spread the diff. case as the diff. assembly is larger than the hole and there is a special tool although it's nothing exotic.
Changing to a five speed gearbox won't alter the engine revs to speed ratio unless it is an overdrive gearbox. Most Ford boxes retain the same 1-1 ratio so will merely give you more gears to play with and not give you a higher ratio.
Personally, I'd have a chat with Billy Bellinger first.
