help on performance decision

If you're just going for some street performance I would not spend money on a new manifold or carb as both are adequate for a 318. Here is where you can figure you CR http://kb-silvolite.com/calc.php?action=comp
The stock pistons are probably dished so the KB 167 would help bring the CR up. Save the money on the carb and manifold and you can have pistons and a 2k-2.5K converter