There was a post on here a couple of weeks ago discussing fitting S13 coilovers to an AE86. It would probably be a good starting point to indicate what you are going to need to do.
Here's the link
http://www.toymods.net/forums/showthread.php?t=11242
Personally, for all the effort fitting s13 struts would take, I would look at getting your RA65 struts modified.
Bookmarks