305=3.78" bore
350=4.00" bore

305s are realyl small bore motors. They make decent torque, thats about it. You will spend a lot of money to make a fast 305 and for what you put in it, if you had a bult a 350, youd be making a lot more power. 305s use really small valves due to the small bore, so they dont get a lot of air in.