Ford Repair: 4.9 cylinder head, exhaust seats, exhaust ports

Question
Attempting to rebuild a 1990 4.9 (300 cu.in.) in line six. My head guy says their are three small cracks (which show only when magna-fluxed) between the exhaust seats and the exhaust ports.  He says it appears that no water jackets are invovled and the valves show no sign of water having been getting to them.  This engine will be run pretty hard in an E-250 van, pulling a trailer hauling windows and siding and tools.  Replacement head is $250-$350.  Question is, are these kind of cracks unusual, or is it safe to use this head?  Engine is fuel injected and had thrown a rod, perhaps from leaking injector and hydraulic lock.

Answer
David-

  I would just replace the head.  No cracks are good cracks and they'll only get bigger as time goes on.  You could probably drive it for a little while before that happened, but eventually you'd be looking at engine damage when the cracks get big enough.  Then it'll cost a lot more than $350.  Hope this helps.

Steve