to do that right would cost more than the hubs, a lot of stresses go through the struts and you don't want that the crack going around a corner..
you be better off getting r31 struts and having the threaded sleves welded on to convert them to height adj with some good shock incerts.
what you got to think about is "why s-13 coilovers" what do you want them for:
height adj - for about 400 you can get height adj kits from Noltec / cusco, $500 a bloke on ebay will send you a front set of struts already made with springs and adj strut tops, $800 and they come with new incerts, These are for and r31 but the only difference to the r30 struts is they are shorter by 30-40mm...you would have to get a set of r31 rotors and calipars but they are bigger and better then the r30 items so thats a plus...