[neonlarry]

I was just going to say the harmonic balancer but someone slipped in before me with that. That and the suffix code are the best, on a Caprice you should be able to see the # fairly easy.
When you get there though unless its really really cheap I'd pull the heads and check for cracks between the bores and also check the pistons to see if they are oversize and how much, .060 over is about the limit on a 400 unless you want to sleeve it. Good Luck I'm still looking for a good 400 block myself.