It's definitely worth 5K. When I look at cars as soon as I see non-numbers matching, I move on to the next car. I know several people who are the same. Not having the numbers will reduce potential buyers if you ever have to sell the car. The engine doesn't have to be in the car, but having it around for the next guy is a strong selling point.
I really don't think it is.
You're right, there are people out there that WON'T BUY non-numbers matching cars. So, you will lose some potential buyers. But, and this is the real questions, are any of those buyers going to pay more than $5k over what someone else will pay for the
exact same car with a date code correct engine? Keep in mind we aren't talking about a matching numbers hemi car here, this is a 340 3 speed car.
I personally think they won't much more than that over someone that will still buy the car with a date code correct engine, unless you've got some kind of bidding war or you find a buyer that just need to have THAT car and it HAS to be numbers matching. Then of course there's the question of whether its worth $5k and the headache of dealing with the jerk that has the engine.
And honestly, I'm not sure how many buyers you're losing. I look for cars that AREN'T numbers matching, and wouldn't pay dime one more for one that was. Unless you're planning on parking it in a museum and using it for investment purposes, its a useless addition to the car. They drive just the same (or BETTER!) with another engine as they do with the matching numbers engine, and can be even more fun because you can actually use them without worrying about screwing up the value. God forbid you have some fun and accidently toss a rod out the side of your perfect numbers matching block.