Yep. puffing white smoke out the tailpipe is a pretty good indication of a blown HG. Does the exhaust smell a little sweet? (Side note: I've been running E85 in the 245 for a couple of years now - on a cold start it will put a rather sweet smell from the exhaust pipe - that made me repeatedly worry about a leaking HG).

There are some test kits that will test the air in the cooling system for the presence of carbon monoxide - which would only get there from combustion getting pushed into the cooling system past the HG. They can be a good diagnostic tool on a borderline case.

But this isn't really sounding like a borderline case, it's sounding fairly obvious. The overheating is a side effect of the hot gasses being injected into the water jacket and overpressuring (and over-heating) the coolant.

Pull the head, check it for flatness, check the block too for good measure (has anyone ever found a warped block? ). Put it back together with a new HG and use proper patterns and torque values on the head bolts.

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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)