where does the gas milage go.

[rbohm]

everything will still work with a 260, just watch the valve size. check to make sure the valves dont hit the cylinder walls, they can be notched for clearance as needed with in reason. usually a 1.94in/1.60ex fits but just. the reason i said assuming you were building a 302 or 351w, is because you didnt indicate what you were building, and if you had a six or a 390, for example, the recomendations would have been a bit different, but not much.