He'll have that set on his shelf for a while. Never seen anyone dissassemble a rear axle at a show so that someone can read the numbers on the ring gear. Case in point ... a buddy of mine has a "numbers matching" AMX, and I would say that car in reality is close to 400 horse. Bigger cam, ported heads, forged pistons with a bit more squeeze ... but it looks really stock.
3.91 gears were used in other vehicles that just B429... Sorry seller, but a set of NOS gears will never make $0.02 difference in the value of any car originally equipped with a 3.91 as long as it has 3.89s in it. That's just silly. If a car is good enough for a 100% judging, it didn't get there because the judges made sure it had Ford gears...