This skinset adds Chris Amon and Roelof Wunderink driving the Ensign N175 and N174 respectively, with helmets and talent files. This will not work without this mod: http://www.racedepartment.com/downloads/f_retro-1975-skins.10001/
The .srs file is included in this download, as it had to be edited to allow the extra 2 ai cars.
Amon should be faster than Wunderink, but both should be near the back of the grid. I have not extensively tested this however.