I'd defo say you've hit the nail on the head there, Blocked rad all day...... Not had a rad weld or such like put through it at some point in its life??? My guess as its a K series is probably yes, and it may of blocked your rad either that or it's just full of "Smegma" LOL.....
K-Seal :scratching_chin:other option try spending £35 quid on some real steel gasket sealant and see if that cures it.
Supposed to be used on nascar engines.
Mike
I would defo remove top and bottom rad hoses and check for the flow rate through the rad...... won't cost you anything and still to me does seem very suspect.