The problem with fitting an overdrive to a non overdrive box is that you'll need a shorter output shaft to mate with the input on the overdrive, and a corresponding outer case instead of the standard tailshaft. It would be simpler to find an overdrive box that would fit, (with an adaptor plate) or stick with a five speed conversion. The four speed plus overdrive unit will also be heavier than a five speed box.
This photo shows how bulky it makes the rear of the gearbox:
That is not the only problem Dave. The gearbox mounting points on the overdrive box are likely to be further back compared to the original, so it is highly unlikely that the gearbox cross member will line up with them. So it means cutting and repositioning the crossmember (best done when in total rebuild mode) , or making up a fabrication to mate the new mounts with the gearbox crossmember.
We used to look out for Humber Super Snipe overdrive gearboxes for Moss Boxed +4s. This was no simple conversion and involved all the problems mentioned above. I remember the first time John Macdonald raced a Super Sports fitted with 0/D and when braking heavily into Tower (first bend at Croft) the whole unit slid forward in the car nullifying the clutch travel and the water pump hit the radiator. It moved by more than a few inches. I will try to dig out some photos to show the differences.
Regarding the OP , I would definitely go for a 5 speed box every time.
