J heads worth?

The 915's weren't much different than the 596's from what I've read. The flow numbers were pretty close. It all depends on how much rebuilding they need as to their value. If they need everything i.e. valves, guides, hardened seats, springs, etc... they can cost a fortune to rebuild so their not really worth much. Most heads that old will need a full rebuild so unless you can get them for $50 your probably paying too much for them but that's just my humble opinion.