Did the mechanic assemble the engine, or just install it?
Is the rebuilder (machinist / machine shop/) wanting the engine disassembled or just removed?The guy who rebuilt the motor has agreed to flush it for free. He rebuilt it, my mechanic assembled it and installed it
UNLESS the machine shop is going to hot tank the block and head I see no reason to remove the engine.
The amount of rust breaking lose that would clog a radiator in 5-6 blocks is concerning though...
Have we tried the CLR and/or a commercial cooling system flush as suggested earlier?