MGB: Transmissions from different years

Hello, how interchangable are the transmissions from different years. I should be picking up a 79 this evening without overdrive. I know of an early OD transmission for sale. Would it fit?

Any trans from 68 forward will fit in any car 68 and forward, but you can run into speedometer issues to resolve. Any trans from 62-67 will bolt into a 68 and newer but you will have shifter location issues and you would need to use the backplate flywheel, and starter from a 62-67 engine.
That may be over simplifying. You would not really want to use a 62-67 trans in a late car because they are worth more t o the owner of one of those cars; sell it and buy a trans for your 68 and on car.
Also, 68-76 (?) were top fill cars the rest side fill. You would have to make arrangenemts for filling if you used the earlier version.

The early gearbox will bolt right in, the fact it is top fill makes no difference, the tunnel of the 79 still has the necessary access hole behind the console. I put an earlier gearbox in my 79, I solved the speedometer issue by buying a 68-74 (early 74, not late) speedometer from the UK. It has the right gearing and is the right size to go in the 79 dash, it just doesn't match the other instruments.

Depends on what you mean by 'early'. If you mean a Mark I hearbox then the first thing you have to measure is the input shaft diameter. They come in two sizes. Second, be aware that the early D-Type overdrive gearbox lacks a syncronized first gear, and is not quite as robust as the LH-Type found on 68-80 cars. You will also need to change a few other things including driveshaft length, engine backplate and a bit of wiring. While there are some advantages to the early unit, I still agree with Chris that you would be best off looking for a later gearbox and leave the early one to the Mark I owners who prefer them.

Thanks for the clarification about the access hole.