Outer Limits 4x4 Board

I spoke to an engine builder who has done quite a few TB42's and he said there is a casting defect in the heads. The entire area below the valve spring seats is far too thin. So obviously when overheated it's the first place to crack. Sometimes repairable, depending on location of crack. He said Nissan now have a better casting. Can someone confirm this? I have discovered there is a superseded part number for a new head which I assume is the new casting. The old number says to refer to new one. The retail price for a new head is about $1800 bare, but maybe able to get one for a little more than half. Same price as a changeover but new! Is anyone interested in doing a bulk buy for a better price? Also, is there anything interchangable between 4.2 and 4.5 eg. the heads?