Hi Bill,
there's no prop made to fit as this wasn't a factory fit, the closest would be for a 2002 onwards 110 which has a p38 range rover diff with a shorter nose than the salisbury so might be closer in length. It's either that or propshaft services and have one made.
I would have left the salisbury on the 110 as it's 4 pin and pretty bullet proof even if its still got drums and put the disco rear on the 90 to give disc brakes on the rear, it's a straight swap onto the 90 once the diff flange is changed.
cheers,
Stevie.