Welcome aboard tommyd!!
50 miles without water is not something that you should live by, especially with the northstars aluminum block and head. 50 miles with out water means it COULD drive 50 miles before the engine is completely fried. They only say that to demonstrate their special overheating mode, where only 4 cy.s run at a time, and the rest simply act as an air pump. So it could be possible you didnt drive it long enough to make it go into the overheat mode.
4500 is fairly reasonable for head gasket, since the powertrain has to be removed. I dont know if it was included, but chances are the head bolt threads are going to need "time serted", which is where they put in special inserts to prevent the head bolts from stripping/pulling again. Make sure this is done while the head is off.
Guys, anything else I forgot??