It should be obvious which one I prefer, but I still like the 400. All the 455 really needs is a cam, some headers, and maybe a set of heads depending on which ones it has on it now. Get us a pic of the center exhaust ports, there should be a number cast into it like 6H, 6S, 6X, 4X or 7H1.. We can tell what heads it has then and if you should swap them or not.
Otherwise use the heads from the 400, they will increase the compression a little and bolt right on. You wont have any problems getting the pulleys and accessories to line up then. Or like Creech said if you can get the stuff that came with the 455 use that, because the water pump pulley is a different size to slow down the pump speed. It will still work with the 400 pulley if you cant get the 455 pulley, just a different diameter, and they should all have the same offsets unless someone swapped in a 67 earlier era front cover.
350, 400 and 455 use the same balancer and flexplate, so no worries there. They are internally balanced and most flex plates are drilled for both large and small pattern torque converters.
The F body mounts from 70-76 wont work on the 77-81 cars, but the 455 should have the bosses to use the brackets the 77-81 cars use to mount the engine. You cant find those mounts in stores, they are stamped steel and all you can get is the frame mount, so dont lose them. Just use the ones you have since most 455s have both mounting bosses. This is what the mounts look like on the 77-81 cars with Pontiac engines. I can get a pic of the 72 455 mounts if you like, they are different.