If all you want is 320rwhp, forget building a block, and just buy the TFS heads, some custom cams, and do the rest of the bolt-ons, and you're there. The stock block is good to over 400rwhp, so why spend money you don't have to. Start doing bolt-ons first, and have some fun, then add the heads and cams when you run out of bolt-ons, and POW!!!!!!, with a great dynotune, you're over 320.
2001 Dark Highland Green Bullitt
12.25@110 naturally aspirated on a bone stock 2V longblock
Under Reconstruction for RS275 Drag Radial Class
Pushrods and Powerglide Coming Soon
If it weren't for physics and law enforcement, I'd be unstoppable.