[unruhjonny]

if you are at a cross roads with parts, please take this into consideration;

A 455 will be relatively easy to sell;
A RUNNING 455 should be SUPER easy to sell.

I agree that an original 400HO car will likely garner more market interest, and better money with a 400 (with period parts on it) vs a later 455.

Now, honestly forget the idea of chasing date codes - I can see from your posts, this is something you are not interested in, and that's totally fair.
If you already have a 400, and if already you have "all the major parts to build a 400 HO engine", why not get that engine built, do a swap, then sell the 455 - as well as any other remaining parts.

At the risk of repeating myself;
Enjoy the car as-is while getting machine work done;
Maybe even find a buyer who will put a deposit on your 455 while the car is together and running - then only hand off the 455, when you pull it to swap in the 400.