It may behoove you to swap on a set of '67 up 318 heads. Your 273 valve gear is a direct bolt on so you will not have any added expense there, and the valve covers also fit. Magnum heads will also swap on but you will need to change the lifters, pushrods, head bolts, and use Magnum rocker arms. The valves clear the bore without grinding clearance notches (barely) but check to make sure if you do this. Either way, you have a lot more choices when it comes to intake manifolds...