i have a pontiac 400. i read that the bore/stroke on the 400 is 4.12/3.75 and the 455 is 4.15/4.21. could i bore the 400 .030 and put a 455 crank in to make it a 455?

The 400 has 3" main journals. The 455's crank will not fall right in. You have to have the 455's mains cut down to 3" journals. This is a very common swap now-a-days. There are a lot of people that sell a 3" main 455 crank for this swap. You get standard bore 455 pistons and you can use the stock rods too. The rods are the same for a 400 and a 455. The 3" main 455 cranks run about $300.

I seem to remember reading that there is a little more of a difference than just the journal size. Something like thrust bearing surface with a different size maybe? Anyway you should be able to find a crank with the appropriate machine work to bolt in like CarNDrvr said.