OK, all fixed. Indeed the head gasket was "gone" between cyls 4 and 6. Have to say, fixing this has been one of the most un-fun things I've done in a while. Between the truck being tall, and the V8 located down deep in the frame, purt near killed me working on it. And worst part was getting the old head gasket off the head and block surfaces. It was like baked on linoleum. The head was bad enough sitting on the garage floor, but the block was a nightmare. Ended up using a hammer and wood chisel to chip it off. Probably would have been less work and far more comfortable to just pull the front clip off so to access the motor. Many, many hours start to finish, but I fired it up yesterday, running better now than I recall for a long time.
One other thing I found, it was mentioned that a plugged up exhaust could cause detonation. Well, that exhaust heat riser valve on the right side was frozen. Not completely in the shut position, but enough to cause restriction. So I guess it's been exhausting through the passage in the intake manifold over to the left side. Not sure if that is what cause the pinging or not, but perhaps the pinging is what cause the head gasket to blow. Will never know for sure, but I replace the riser valve with a "dummy" version with no valve in it. Here in California we don't really need this, it don't get cold like back east.
So now that the truck is running, will be on the lookout for a 292 six to replace this 400 with. There is one up in Sacramento area I may go have a look at. But at least I can take my time about it, have a running truck. Yay!
-John
