Are 302 heads on an older 318 plug n' play?

Almost. you have to plug the smog holes under the exhaust ports. Use 1/4-20 set screws with threadlocker & you'll be set. you may have clearance issues with stock exhaust manifolds. Ask me how I know? anyhow a set of headers may be called for as clearancing the stockers leaves far too little material to be comfortable with.