1972 318 need more power

Sounds like your on a budget. Easy and cheap is to shave your existing heads,maybe even up the valve size while your at it,and swap out the cam for something a little more agressive. Comp makes some good ones,as does Lunati.

Also not mentioned so far is the "302" casting head. Used from around 1985 to 1990,have a heart shaped combustion chamber and swirl ports. Will bump comp and provide better burn.Good port velocity,but not a ton of air delivery in stock form. Probably a better port than you currently have,and will be an easier and cheaper direct bolt on compared to a magnum.