The 455 had a bore that is only .030 larger than the 400 and 428, the difference is made by the crank. The 400 has a 4.125" bore & 3.75" stroke crank, the 428 hasthe same bore but a 4" crank, and the 455 has 4.155" bore & 4.21" stroke. The 350 has a 3.75" crank as well, but with a bore smaller than 4".
Externally they all look the same provided the year breaks are compared. Pre 65 is one set, post 67 another for different reasons, and some of them back in the 50s had reverse cooling. But for the most part, they are very interchangeable. They all have 6.625" rods, the 421,428, and 455 have 3.25" mains while the others have 3" mains. They share almost everything except for the main bearings, cranks, and pistons. There are some variances (mostly in the heads), but for the most part they use the same stuff.
Getting 440-480hp from a 400 isnt that hard, the easiest way is to stroke it with a 4.21", 4.25". or 4.5" crank which will increase displacement to 455-496ci. Another easy way is to increase compression and airflow, get more air in and work it harder. Doing it with pump gas and not stroking it takes more effort and requires good heads, usually aftermarket aluminum heads.
If you are going to get a crate 400 get it from Mr Pbody, Butler, Kaufmann, SD in Canada, Spotts, Ken Keefer in FLA. There are a couple others too, but those come to mind easy for me. You will get a quality build that will make great power and last, plus it will be an excellent foundation for future performance upgrades. A reman will have the lowest cost parts in it, cast pistons, cast rods, and a stock replacement cam and related valve train parts. I like to order the parts, get the machine work done, and assemble them myself, that way I know what is in there, and I can pick the best parts I can afford. It takes longer but I know whats in there and its my combination, and usually I can build them far cheaper that way.