First thing I'd do is get rid of the clutched fan and find a six-blade truck fan. Flush the system with lots of volume of water, a garden hose isn't enough. Other things that make 'em heat are retarded timing and lean mixtures, but I imagine you already checked that.
Not really clear by your posts if you rebuilt it or got it rebuilt,or how it ran in the chicken truck. Could it be the head gaskets backwards or corroded through, so the water is only circulating through the front of the engine? That can happen if it had straight water, and steel gaskets. Heating that fast, and lots of steam at the cap, would lead me to think there's compression getting into the cooling system.
Just a few things to think about before investing in a rad that may not cure the problem.