The rear of the heads have coolant passage coverplates-could be leaking from one of them or their studs?
Glow relay is often mounted to a rear stud in that area and would have needed some pipe sealer when re-installed.
My 6.5 leaked from the block heater, caused by it shorting out. When I put a lower hose heater on it, I used some of that Bars stop leak powder and it doesn't leak antifreeze now, motor oil and power steering completely different story however. Might try it, its 1.62 at walmart, but you need about 3-4 bars of it, but it worked pretty good. If its a big leak, might not try it.
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