the cost of a new cylinder head gasket is probably around 100 dollars, not more than a couple hours to remove too. least for the c11 its 120 usd here, so it'd definitely not be that much to pull the head, when it'll likely give you a much better view of what's going on hey.
the radiator group will have to come out to get the cam out i believe. there was a cam failure on the d6t at another project, just befor i qualified. i got moved there just after it broke down, it was also because the valve bridge kept jumping off. the push rods were ok, and when we set valve lash it would be in spec, it fell off twice more before the cam follower revealed itself to be the culprit. it would turn onto its low spot (i assume) and jump off, then settle on its good face and leave us confused and without an explanation. so it was sent to our workshop n fixed there. there was no SOS sampling to be heard of and only at the end did the metal reveal itself in the oil.