If you want power get an Oldsmobile!!
Seriously though the response about what bolts up is probably the best. I would go this way unless I was building a machine from scratch, then I probably would use something a little more uncommon. How 'bout a Caddy, flatty or Olds, maybe even an early Hemi or a Buick nail head? It all depends what you're going for. Are you looking to just make it go or are you looking for a motor that stands out from the crowd?