First of all, a tear in the driver's seat isn't going to drop the value down by 2k. If you're that picky you might as well shop for a new car. I would be more concerned about rust and the engine.
Second, you can't compare you car to it because a) It is 6 years older (regardless of mileage) and b) It is a GT which is worth MUCH LESS than a Notch. Original 5.0 Notches are very rare (especially in that condition) and they go for 5-6K. Third, a 4 banger notch converted to a 5.0 isn't worth crap because it is not ORIGINAL. I didn't know about the transmission so I think 4-4.5k would be a good price for it.
If your 87 GT was in 5x better condition it must be brand new

because that notch is in pretty good condition for it's age.