2 1/2 inch exhaust on a 318...

OK the reason I had 100 hp difference was b/c I am not very sure of the compression or the type of head. With the rate of gas prices I dont want to have 10:1 - 12:1 compression and have to run top grade. I am really thinking between 8:1 - 10:1 so I can get low grade. It is already 3 dollars a gallon where I live and top grade is up to 3.20 - 3.30. So really I am thinking between 400-450 hp on the engine. So 2 1/2 inch pipes with chambered mufflers? Thanks for the info!