I sometimes think that Minor owners are perceived as pushovers.....some don't like it when they find out otherwise
The problem with gearbox swaps and Minors is the steering rack. Most Minors, with 1275 engines, have Midget spec engines, as the back end fits the BMC gearbox better. Taking this a stage further, the Ford gearbox conversions are designed around the BMC backplate and the special bell housings have the cut out for the steering rack. The question is (And I don't know the answer) does the Sierra automatic box fit the same bell housing as the 5 speeder

If it does, then that could be the way to do it

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The Traveller certainly is a useful car and funnily enough, I saw a Mercedes (190 size - if they still call it that) estate yesterday. The boot floor area didn't look a lot bigger than the Traveller and when you factored in the way the boot lid sloped on the Mercedes (It would be very aerodynamic going backwards), I reckon the boot was smaller, even though it was a bigger car. It was a bit like when my father traded in a Mini Countryman, for a new Escort estate; the Escort was the larger car but the Mini made better use of the space it occupied.
Rules exist for the obedience of fools and the guidance of wise men.
MG Midget 1500, MGB GT V8, Morris Minor Traveller 1275, MG Midget 1275 & too many bicycles.