In my opinion, it would be worth more with the 400 in the car and with the 326 on the side. You have to factor in the grins you will get with the added horsepower too. After all, you need to get all the pleasure you can out of the car to offset all the repairs you will be doing and parts you will be buying. I have gotten very good money for a numbers matching 400 car with a 455 in the car and with the #'s 400 on the side. I actually did better on that car than on a similar numbers matching 400 car with the 400 in it. With that said, maybe you should think about a 428/455. If you are thinking of resale your 326 car would be worth the most with a 455.
Jim