Hi Andy, thanks for clarifying this. No a BTR diff won't fit a Salisbury axle.
Andy, you mention about the danger of fitting a wider axle to an 'early' car. There are a couple of erroneous articles in magazines and books that say this. A number of 'library/bar room' experts merely restate this nonsense.
1976 4 speeders were fitted with the wider five speed chassis. My car was fitted with one and I am almost definite 1975 cars were as well.
There are two ways of determining the type of chassis fitted to a Plus 8:
1). Measure the width of the chassis and the axle;
2). look under the car, if the chassis has a cross member that sweeps below the clutch bell-housing then the chassis will be an early narrow chassis; if there is a straight U section cross member it will be a wide chassis.
My apologies Andy if this comes across terse but I hear this nonsense repeatedly stated and it is wrong. Even the factory has probably forgotten this fact.
Would any 1975 and 1976 four speed Plus 8 owners like to look underneath their cars and verify?