If you port and polish your heads yourself, its just time and little money. The issue is the condition of the seats and guides. If you have to send them off to a machine shop for that work, you may save money by buying a new set of heads. I used to port a lot od 906 and 452 heads for people, seldom get asked anymore, cheaper to buy a cheap set of new heads. But yes porting those head are well worth the effort as far as performance of your project is concerned.