When it comes to the R31, the guys over at R31 Skyline Club know it better than us lot.
See here:
http://wiki.r31skylineclub.com/index.php?title=5_Stud_Conversion
http://forum.r31skylineclub.com/?topic=138699.0
Can use S13/S14/Z31 bits and bobs, those links should tell you everything you need to know.
Also, welcome the the forum.