Derailleur Compatiblity Question

[FierteTi52]

I'm helping a buddy upgrade his early 90's Shimano 600 8 speed drive train to Ultegra 6500 9 speed. His old 8 speed STI levers are shot. He bought the Ultegra 6500 9 speed STI levers, chain and cassette. I'm curious if the old 600 (grey colored) rear derailleur will work, or if he needs to find a 9 speed 6500 rear derailleur. I'm aware that the new Ultegra 6600 will work with 9 or 10 speed but it is quite expensive. And of course the 6500 stuff is hard to find. Any help would be appreciated. I'm sure someone has messed around with this.
Jeff

[gdw]

I've used the older derailleurs with 9 speed bar-end shifters without any problems so they should work with STI levers as well.

[bluehorseshoe]

beware -- the spring tension in some of the 8 speed r. der.'s is too high to work nicely with 9 spd sti shifters. should be no problem with dt shifters or bar cons, though.