I really like to deviate from stock when its an improvement that doesn't take away from the classic look and makes the car more enjoyable to drive..

For my car I used the 11 bolt WP, Internally regulated alternator, small body HEI system, 15" ralley rims, 455 with Long branch manifolds, Guldstrand mod, eaton posi, damplifier pro on the floor. From an appearance standpoint it looks like a stock 400 car inside and out.

Like blue bird said mods will date the car but the factory look will never go out of style for the car and IMO will help hold the value.

You said the motor is locked up. Try pouring each cylinder with marvel mystery oil and let it sit a few days. Even if it does free up you still might be better off just rebuilding it with better modern parts.

If it were mine......I would......... drop a 428 crank in if the motor, cheap forged rods, Icon pistons, send the carb to cliff, have the heads ported and go with a 5 speed trans. Get the Long Branch manifolds and drive it like a rental car.

Since you have to replace the 1/4 and it sounds like it getting painted anyway I would just stick with the factory color unless you hate it. Its much cheaper not having to change the jamb colors and as I said above will help maintain the value if you ever where to sell it.

If the motor is out. Look over the sub frame and see how the bushing looks. It would be a good time to detail it. You can do this as cheap or expensive as you want. if you end up pulling the sub frame make sure you measure the heck out of it putting it back and look at the youtub on how to do it correctly. This is where scope creep can really start hitting hard.

I would replace the carpet, and spray dye the seats/door panels if they are still in good shape. You would be impress how well this works to revive an interior.

if the trunk and floor boards are all solid but just surface rust you could coat them with POR and be done with it. This is another area where you can spend as little or as much as you want.