ok we've hit a snag, yours dont use circlip groves...
No clue what model they are but they've clearly got a threaded body & it's looking like your spring seat is wound all the way down (unless theres some more thread under that black bandy thingy?) Modification is probably out of the question now, though double check with a quality suspension place. Might have to start looking at buying something new again.
Have you considered buying off yahoo auctions japan? lots of options for you both new & used, simply find something you like then contact an importer. I've used importmonster & Jesse Streeter, both excellent service.
For example: aragosta, HKS 4way, HKS hipermax 2, Ohlins, Top Secret, Apexi N1
All will sell for between $1000-$1500 (roughly, could go either way) & need shipping + fees on top of that, say $750ish. Much cheaper bargins can be found if you have a good look. I've got the hipermax 2's in my z32 & find them fantastic, all the others are highly reknowned & are high spec coilovers.
cheers, Tim