No civilian Hummer. There are people who replaced the factory engine in their hummer with p400. The p400 was designed and built after civilian Hummer production ended.
are you on the Hummernetworkforums.com ? What screen name if so? There is a few Hummer owners from Russia on that forum- one guy does AMAZING rebuilds and fabrication. He sells bulletproof windshields almost as cheap as regular windshields. Makes all kinds of stuff.
the head crack like that comes from one thing-heat. The manual is WRONG about acceptable high temperature. Keep the temp under 220f at all costs. Even 210f is damaging over the long haul.
Our hummers are garbage for getting rid of heat. The aluminum radiators help a little but not enough. I had a custom radiator built, and a friend that I met on the other forum had his built the same way. He has a ton of miles on his now both on road and offroad and the radiator does great.
4 row dimple core. Before that he had a secondary radiator in the bed to get rid of heat and is much happier with this.
You seem to have some descent rebuild skills knowing how to deal with rusted in glowplugs and such. The expense of p400 heads is painful but if you can afford...
You also might look into of you get them from TwistedSteel, he does the porting, coating, etc. obviously more money on top of the cost but it is wise to know your options.
Otherwise get new optimizer heads and do the steam pocket inserts would be my next suggestion. That is my current game plan unless I accidentally fall into enough money to buy fully done p400 heads from him. I doubt it, but I can dream- right?