440 lo comp rings worn?

hello there, just a quickie, have a 66 casting 440 lo comp motor in my four speed barracuda, has 906 heads and a real slug,has run hot and overheated due to air leak on inlet manifold, since cured that, runs fluffy up to 2800 pm then clears after that and runs ok,thats a carb prob that being dealt with seperatley lol, two things, first my pal said it needs new rings as "breathing heavy"ie bit of pressure back thro tho oil filler caps however car does not smoke at all or burn oil,secondly after around 20 mins the car feels like it losing somthing, was suggested the vales tightening back into head, ie when ran hot/lean probobally done the seats in.he suggesting full rebuild, i know the cam is worn, i was hoping to pick up a set of worked heads and cam and simply bolt on. could what i am saying make sense? appreciate there are seperate rings doing seperate things on pistons but am not using any oil, cheers in advance, nathan.