someone help me with my slant 6 problem

70 Valiant is correct. You need to fix your radiator. Ill bet it is plugged. I learned the hard way that the "hot tank and or other cleaning" will loosen up a lot of debris in the system. Then when you put your rebuilt motor in and start it up ,Woosh all that crap is now plugging you radiator.
I now put a strainer in the hose for the breakin period ( I have a hose I cut in 2 and inserted a s/s basket strainer then put back together) Then I put a regular hose back in after the breakin.
My02.
Frank