$800 a pair ready to bolt on isn't bad, but the shipping and core return to/from HI could be cost-prohibitive
When looking at rebuilt heads, always inquire if ALL exhaust valve seats have been replaced with hardened seats and if all valves have been replaced or if it is a mix and match rebuild. Most head cracks usually involve exhaust valves, and center cylinder failures are common due to higher heat in that area. I would also make sure all fixed cracks are magnifluxed and the heads come with a complete replacement guarantee.
In my experience with other engines, starting with used head cores for head rebuilds usually always creates smaller chamber volumes as the heads need to be surfaced flat, so a thicker gasket is required, otherwise you will increase the final compression ratio. Make sure either the gaskets are supplied by the builder or they tell you exactly which thickness gasket is to be used so you don't end up detonating yourself it death.