From what I remember from my research on my 200tdi conversion to a series 3, the series 3 gearbox will bolt up BUT you have to remove a couple pins from the engine and also drill/tap a couple more holes. Not as scary as it really sounds.
You'll have to check defender/disco differences in injector pump locations and see which will fit the series 1 chassis best. I used a disco engine with defender manifolds and turbo for ease of use. I'm not familiar with seires 1 though
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