To stuff a big block in there will be a lot of work. You can grab a 360 for cheap and build it to run with or better than the big block for less than it will cost to buy a big block and install it.
Stay small block.
I am a home builder and I would say there is alot of things that would very my prefrence. Where are you building it at, COST, restrictions for your neighborhood (if in one), and weather and soil types. Here where I am at I would go with metal.
So what if the SS will run high 12s, I saw a SRT 8 Challenger with a mopar cold air kit and no other mods run 12.54 2 weeks ago and they look better doing it.