The 400 / 455's need the correct harmonic balancer installed, I don't think you will get the early front cover to work with the late balancer. You will need a 69' or new front cover and water pump for the big engines. You will also need a big radiator to cool the 455, it has less water capacity then the 389/400's so they tend to run a little hotter.
How about just painting up the new motor to look like the old one? Add vintage valve covers and air cleaner and moet people wouldn't know the difference.
One thing to think about, it cost just the same money to build a 455 as it does a 400, 389 will be more then both due to demand of parts, 421/428 cost even more money then the 389. You can now buy stroker cranks for the 400 block that will make 455 or bigger engines out of them. The 400 block had stronger webbing and more metal in the lower end. You can also get 505 inch Pontiac blocks that look stock. The stroker kits (crank, rods, pistons) are pretty cheap compared to finding 455 which are getting scarce. Joe
