These look to be good final drives Sambo. I did some calculations using:
http://www.mustanggt.org/gttranny.htm
and
http://www.kabamus.com/garage/gears.html
With my tyres 275/40/ 17 and my final drive is 4.1 by the part number
With the standard set up I should do about 2300-2500 at 100km/hr (my iPhone GPS measurement in the car tends to confirm this too).
With the S15 helical final drive that would seem to come down to 2000 rpm.
Of course that could make a hole in acceleration (perhaps the THE key aspect of these cars) unless more power was produced thru engine mods and/or higher rpm (engine life?)
I am thinking that I will buy the S15 diff and shafts to put aside for now in case they become scarce. So I am really keen to see how yours goes. Warren of course has done this already (above). His driving experiences will be interesting to hear, although he has a different motor and transmission.
Merv
Merv