Why exactly does the OP want 500mm chainstays? I guess if he is heavy, longer chainstays could provide better weight distribution between front and rear wheels, or maybe the longer stays would allow for more heel clearance with rear panniers, but other than these two possibilities, I don't see the reason for 500 mm chainstays.
Maybe if he asked about chainstay length on the framebuilder's forum, he could get some advice from people who have designed and built frames.