Not sure about the 74 classic engines noise issues. But I seriously doubt that "NAV" equipment would be a factor. GPS is about as accurate as you can get, and I've seen some classics fitted with Honeywell FMS, the same units, or nearly identical, as we have in the -400s. I've also seen a moving map TV mounted in the pedestal forward of the throttle quadrant.
Keep in mind that a number of classics are fitted with early -400 engines, [CF6-80C2B] which would still be noise compliant beyond 2011. Some classics still rolled out in 1990, two years after the first -400s came out of the shop.