47-55.1 250 coolant sludge or hose lining breaking down?

[leegreen]

The 250 6 in my '52 is a good runner from a 78ish c10.
It keeps turning the coolant brown, It was flushed before I installed it and I changed coolant twice last summer, once to try and get it cleaner, then again after the water pump took a dive..

Radiator is old, but appeared fresh rebuilt in a parts truck I bought decades ago, it sat empty until last year. I flushed it before install and it seemed clean inside. Heater core and all hoses are brand new.

truck does not leak or use coolant or oil at least not noticeably. no evidence of oil in rad or coolant in oil

I have the radiator off to fiddle with front sheet metal alignment and flushed it.
I kept getting these crumbly chips out of it, they are up to finger nail size:

They are rust colored but not rust, not magnetic

Flushing the rad, it ran brown for quite a while until all the sludge was washed out

The rad hoses are new Dayco flexible hoses, both are coated with the same hard crumbly stuff inside. The upper (outlet) hose is worse

and lower


the thermostat is stuck open and will be replaced but the housing and neck are both clean, no sludge.

Has anyone seen this before?
Could it be some kind of hope-in-a-can leak stop gunk that was hardened inside rad or block until I started running it?
Suggestions for removing it? it seems flushing just carries away the loose stuff