My experience with diffs.
If you are going for a 3.62 to replace a 3.92 Zetec diff, you are only saving 300 revs at your cruising speed of 70 mph.
I think one of the great advantages of the original diff is the range of ratios available. Burtonpower list 9 different ratios, from 3.54 to 5.29.
Several year ago, I fitted the complete Zetec Sierra brake system to my Spyder chassied +2. However I retained the original diff as my chassis was a Mark I and would not accept the Zetec diff. Spyder made up a set of custom drive shafts to mate the original diff to the Sierra hubs.
The original diff in the Plus 2 was a 3.9, with the 4 speed gearbox. Over the years it went to 3.54 with a T9 5-speed gearbox, which then acquired a long 1st, then recently to a 4.55 for competition use. The engine is much modified and is presently at 1860 cc / 170 hp.
I made up a standard diff brace very early on, and recently modified it to attach to 3 ring bolts.
As the car is road legal and I drive to all events, the 4.55 was very limiting for road use even with the T9 5 speed so I am at present replacing it with a 3.9.
(Great recent mod I learned here was to cut of excess casting metal under the diff nose, makes diff swops a doddle ! )
One of the original diff output shafts died quietly from fatigue failure while returning on a 265 mile trip from a hillclimb last year. Both output shafts were replaced by Tony Thompson billet shafts. I estimate the original shafts survived about 700 racing starts.
I fitted a Quaife ATB diff quite a few years ago, which is changed over with the different ratios.
As I have just won the 2019 Irish Hillclimb Championship (Historic Class) for the second year running, at my age I look forward to taking it a bit easier next year !
