Bill,
Firstly your Moss Box and elektron bell housing are extremely valuable items, so if you do end up making the conversion, then please preserve them very carefully , or sell to a very hungry market and get full value for yourself.
Secondly there are a number of 5 speed conversions available, but not all are commercially available off the shelf. The Ford Sierra box (sorry I don't know the designation , is it a T5?) is certainly one of the more common, and there is a fair choice of ratios for it. In Australia a Toyota Supra has been used and there are undoubtedly others as well. However you need to ensure you get one with a ratio of less than 1:1 in 5th gear , or you will be back where you started.
I think I am correct in saying that both of these conversions involve moving the gearbox cross-member , so you have now changed the car irreversibly.
The Moss overdrive option might seem a better option but even here there are issues that you should approach with both eyes wide open.
Firstly the only overdrive unit that bolts straight onto a Morgan Moss Box comes from a Humber Super Snipe , and those really are like rocking horse manure these days. Because the overdrive unit bolts on the back of the box it is now much longer so the bell housing must be cut and shut, no easy job that as welding magnesium allow is not a common skill these days, and you have now destroyed any value in this component. Secondly again you are forced to move back the gearbox crossmember, and thirdly you have to rig up some sort of fore-and-aft linkage because the whole unit moves drastically under braking (don't ask how I know)
Unless you have a very powerful uprated engine, the aerodynamics of the flat rad will limit your cruising speed to about 75/80 mph , at which the engine will be turing over at about 4,000 rpm. As others have said , I think it would be much easier to change the ratio in the back axle BUT as yours is likely to be a 3HA unit I am not sure that there are any new crown wheel and pinion spares available for axles of this vintage.
Good luck and let us know what you decide to do.
