Triumph Repair: Upgrading the 4 speed transmission???, british car clubs, overdrive transmission

Question
Howard,  I've got a 75 Spit with a 4-speed and would like to upgrade to an overdrive one.  I obtained a Type J Overdrive unit that was totally rebuilt, and have been looking for an overdrive transmission to mate it with.  Talking with a friend who is into MGBs, he indicated that it should be a fairly simple bolt on to the regular transmission, wiring it up, getting the proper mounts for the overdrive, and cutting the drive shaft down to accommodate the overdrive.  Is this correct, or do I need to find an overdrive transmission to use with the overdrive unit?  The money isn't the problem, it is finding an overdrive transmission.  I'd appreciate your help on this problem.
Many thanks in advance!!

Bruce

Answer
Hi Bruce,
No, you can not just bolt the OD to a standard trans. The "output shaft" (main shaft) is different and the OD needs that OD mainshaft. If you could find an OD mainshaft you could install the mainshaft in place of the standard mainshaft and then install the OD OK.
Many British car clubs have Spitfire owners who gather parts cars and that is then a possible source of a OD transmission.
If you have deep pockets you could probably get all you need from Moss Motors in CA.
Howard