I'd like to put a 105 derailler, originally designed for 6-speed indexing in the late 80's, on my dad's bike that has a 7-speed RSX drivetrain. Do Shimano 6-speed deraillers have the same cable pull as 7-speed (and thus be compatible with 7-speed shifters and cassette)? I've not been able to find any solid information on this yet.