Yes, modern shoes will work with perhaps a little alteration to the calipers. I recently put some Kool Stop salmon Dura brakes shoes into a pair of old Shimano Exage brakes (from the late '80s). I had to grind out the slot on the brake arms just tad wider (hardly anything really, in fact, some of them very nearly fit). I used a 1/4" drill bit and just ran it along the sides of the slot. The issue is that older brakes were designed for a threaded bolt to fasten the shoe but the newer brakes/shoes have the threaded post as part of the shoe and the fastener is a cylindrical nut that must be inserted into the slot in the brake arm.
Those Shimano 6400 brake shoes will likely work w/o any alterations. I'd go with those since you're selling the bike.