I found a 1991 Trek 8500 in the garbage last week. It comes with the Shimano XT 7 speed grouppo (RD-M735 rear derailleur for this question). This frame has a 135mm dropout.

I also have a 9 speed wheeleset with a SRAM 9 speed cassette and PC-59 chain, plus a SRAM 9 speed Attack shifter. Question is, can I use this older 7-speed derailleur with the newer 9-speed drivetrain? any issues I would need to resolve? Or should I get a used XT rear derailleur (RD-M750) and be done with it? Don't want to spend anymore $$$ than I need to.

Also, will the older XT crank be compatable with a nine speed chain?