Take the dissembled heads to a DIESEL machineshop. Have them Magnaflux it, pay them for that and to grind out the area of the crack and let THEM tell you if it's fixable. If they pronounce it repairable, they will also Magnaflux the rest of the head before welding it. Also, have the other head magnafluxed at the same time. Both heads were made by the same foundry, at roughly the same time, and if one head has issues, the other one may have issues too.
If the heads were "poured cold," the molten iron coming in from multiple sprues may not have been hot enough for the metal to completely fuse together before cooling. That is what it appears to me as. But I am not actually looking at the head.
Those shops weld a LOT of cast iron and have to be good at it.
We have a great cast welder at a local Diesel shop here in the Phx,Az. area. He welded up a pair of Olds 455 heads over 3/4" higher on the exhaust, and a half inch on the intakes so we could raise the runners. The same guy is still there. We use them often.
It's worth the $$$ you pay them to know if it is fixable, so you will never have to second guess your decision.
We, as most shops, will credit the cost of inspection towards the total cost of the repairs, if it is repairable, they would have to do that first anyway.
Know you will have to:
Surface, touch up the valvejob, and lightly ream/check the valveguides, after the welding.
TIMINATOR