It doesn't make sense, does it? Looking back at the build pictures you've got the radiator, plain formed upper & lower connections to the engine, coolant recovery tank back on the firewall with hose down along the passenger frame rail, and no heater hoses, right? How did you plug the heater connections on the water pump? I could not find a picture showing enough detail to tell. It doesn't sound like your ride was rough enough to flex the frame, and even if it did your upper hose looks like it would allow radiator movement without any pull. You don't suppose you might have had a thermostat stuck closed, headed into a temperature excursion coincident with your off-road excitement, or maybe in the two miles to the cruise? The only thing that makes sense to me is the stat stuck closed, got hot and burped from the water neck connection (slightly loose), and you were seeing coolant running from the top of the engine but it had already relieved pressure and stopped burping. It's a stretch, but I can't see anything else that makes sense to me.