53 8BA - Vanishing coolant

[RuralTowner]

Sat for 40 years & recently got vehicle in near-to fully roadworthy condition. Was having an issue that while sitting & idling the temp would slowly rise, it would rocket to 180 after only a couple miles of 45mph. This with no T-stats or anything else to regulate flow. It didn't use coolant or leak. Could sit for weeks & still be full.

Filled block with vinegar & got it up to temp then let it sit for 3 days. Resulted in making an original brass//copper rad weep like it was at Henry Ford's funeral. Top back off with vinegar & idled for more temp until a tank blockage put a stop to that. Flushed & back-flushed block both directions on both sides with engine off & repeatedly flushed w/ hose stuck in the head while engine running. Did this until only had clean water coming out. Went & stuck an aftermarket aluminum in place for now. Now it runs REALLY cool down the road...too cool as it BARELY hits 140 when sustaining 50mph in 110+ ambient air & takes about 6 miles to reach even that. After a solid 14 mile one-way run (limit of the local road I can test on w/o hitting the highway) it will then hit 180-190 during idle rather quickly then plummet to 140 again after only a couple miles once at speed again. Cooling a bit TOO well...though not a great thing it's preferable than baking itself just sitting. Going to try the washer method as a flow regulator to see how that works. Have a couple thin copper plates w/ ~5/8in openings (can go bigger if needed) patterned from the outlet that will get sandwiched between gaskets.

Issue I've found now is that I'm losing coolant when all prior low-speed tests prior to the flush never lost a drop. After about 30mi worth, parked it, then next day checked & it took a gallon + to top back off. Just sitting & idling from cold I don't get any bubbles out the radiator, but after the vinegar flush it sound like the old head gasket may have opened up somewhere just enough to suck coolant? The engine used to use just straight water so it had plenty of rust that vinegar would've no doubt "cleaned" even at the water jacket seams. But no external leaks from anywhere.