I almost always agree with oldskool on questions like this, and he has given you an example of a good starting place. We can provide more specifics as we learn more about what you really want.
You bought a fairly low mile, nearly original, nicely restored 1973 455 T/A 4 speed. You paid for it and can do what ever you want, but you might want to be careful not to loose your investment. If your car is # matching or mostly so, replacing any of the parts on the motor or car and not keeping the removed pieces would be a $$$ mistake.
If # matching, you really might want to pull that motor and save it. Then buy or build one and drop it in.
If it were me, doing it over with mine, I would set the budget and HP level first. Your at 250HP stock, 350 -450HP is fairly easy and reasonable money, not too hard on all the other drive train parts, so leaving them stock at those levels is a reasonable option. Somewhere at or above 450HP nearly everything else needs to be upgraded too.
On your stock 455 the really weak areas are oiling, connecting rods and pistons but you can still make 450 HP with them in place, if you want, thou some or maybe most would recommend upgrading them as well.
The easy recipe to add power would be new heads, cam, intake and exhaust, with a tune on the distributer and carb or replace them too. The problem is new or rebuilt heads may result in oil consumption as the old piston rings can't keep up to the seal from the new valves in the heads. So then you are back to doing the short block and should upgrade its weak areas while at it.
So back to the question of budget and HP
Do you add 100HP for something like $500-$1500, 200HP for $2000-4000, 300HP for $7000-10,000. These prices are by no means exact, just kind of ball parks. If you're not doing work yourself and want a really good job, their likely low, especially at the higher HP levels as you really need to upgrade other things too.