Thanks for your comments Paul, it seemed to me that some of the comments above were over complicating the subject at the level of information requested.
To comment on the diff ratio question, a 4.1 ratio will raise the engine revs for any given road speed, possibly reducing the potential top speed of the Car. There will be no detectable change in any other characteristics of the vehicle. The only time that I have used a 4.1 ratio was when racing on Oval Circuit's where it was desirable to pull as many revs as possible, because oncemoving, gear changes were not required. After all, the Ford 4.1 diff was typically used in Anglia Vans. The most desirable diff for Race Cars at the same time was a 3.7 if my memory is correct.