Anti Freeze turned brown.

I bought my 67 from the original owner. he knew he had 2 very small leaks in the radiator and a leaking freeze plug. He had added a bottle of barsleak about annually and a weak system cap ( 7 lbs ).
When I removed the rearmost freeze plugs from both sides I found nearly fist sized chunks of material . I had to break them up to get them out the freeze plug hole.
Good garden hose flush through those freeze plug holes and a new radiator, proper cap, all good. 6 years now and the water stays green.
I'm sure a lifetime of conventional flushing wouldn't have cured the discolored water issue.
You should also know that if you drape the heater hoses over the fender and hose flush the heater core in one direction until it runs clear, change the direction of the flow and it will be brown again. To change flow direction several times will get it all clear.