Guessing the bike is from the 1993-1995 period, as this is when the combination of threaded headsets and 1 1/8th" steerers were most common, at this time, stems were in the 120-150mm range, will you be keeping this, or getting something more upto date, like 70-90mm? this will have a major effect in handling, probably more than a few mm on the ATC,
What about tire size, BITB, 1.9 was common, 2.1 was large, now 2.1 is small, all these factors have an effect on the bike, have you thought about them?