Isn't there something about big valves in that sub 4 inch bore also? valve shrouding etc.? This is the thing for me its always a reengineer a turd game with 318s. the only reason they were good to me back in the day is they were cheap and plentiful direct bolt in swap with a 340 if you needed a temp motor while rebuilding the 340. they made millions of the van/truck/station wagon motors.