The biggest determinant, assuming a triple crankset and 3-position front shifter, is likely to be if it has a MTB shifter or Road shifter. Seems like in the 6-speed era MTB shifters were the norm, so you'd need an MTB front derailleur. It also needs to be a front derailleur made for the size of the chainrings. A FD made for a 42/32/24 typically won't work for a 50/40/30 crankset for instance.
6,7,8,9 speed probably won't matter.