Head decision time

Any motor will benefit from a bowl cleanup and a port match. I'd use the 273 heads and remove the factory ridges that are cast into the bowl area, you don't have to go crazy, just smooth them out to be flush with the rest of the surfaces in the bowl area. I also gasket match the intake/exhaust ports and smooth everything in following the factory contours. Besides the cost of buying a cheap electric die grinder from someplace like Harbor Freight, this is free horsepower. Also use the thin Mopar composite head gasket to get optimal quench with your factory type pistons. Unless the valves are in bad shape, I'd stick with the factory valves for cost and performance reasons. You get better flow with the bigger valves (assuming you bowl port to remove the flow restriction), but your motor build will likely not notice the valve size difference. I'd stay away from the SP2P, too restrictive, find a good aluminum dual plane at a swap meet, or use your factory 4 barrel manifold. 318s aren't the baby engines everybody labels them as, it's only 9 CID smaller than a 327 Chevy motor, and those motors came factory with 2.02 valves, solid lifter cams, etc. Try to get compression in the motor and let it breathe. I built my last 318 just like any other motor I've done (cam, ported heads, etc.), and it ran VERY strong, bottom end up to 5500.