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If you are looking for stock you can purchase a complete rebuilt set from parts store like Advance for about $200 ea. They have 20% off right now so that will help some. I would think the scrap yards are full of 302 cores and engines......just have to get out and make a few trips or calls?
You can use heads up to like 1995 after that I think they change the exhaust ports so your manifolds may not bolt up. So finding a set of heads should be pretty easy from a 5.0 H.O.
I would stick with mid 80's to early 90's heads. Mid to late 70's 302's and 351W's were lower compression and some had wierd hook-ups for the smog pumps. As far as I recall, 1976 being the worst.