302 heads

For dollar per hp I don't think it'd the best up grade. Unless there low mileage and don't need rebuilding and are free or pretty close to it. They will only give you 5-10hp at best unless you mill and port them, I don't think you would get a full point of CR with out milling.
Cam, 4 barrel and headers and gears will give you the best bangs for the dollar. Unless your trying to run a much larger cam than stock cr can support money probably better spent else where.