[quote=DaveW]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.
Yes there would need to be fabrication of a longitudinal member between the 2 existing cross members to mount the OD unit, there is also a certain amount of metal fabrication to fit the T9 5 speed box
Regarding the OP , I would definitely go for a 5 speed box every time.
The 5 speed conversion is well proven and a good conversion however I think it worth looking at other options to achieve the same result, any engineer worth his salt would do so and its always easy to copy whats been done before.
