They may have a J on them, they are basically the same. Effort just depends on what you want to do. The 596's have some advantages over the 915 J heads.
IMO, put in a 1.94 Chevy valve, blend them in your 596's and you have a really good working iron head. Not hard to get 400 hp out of them with the right choice of other parts. A really mild build will get 350-370, XE268H, performer rpm/LD340, 750DP, headers.
