Put me down for airflow. Is there a whinch or big pushbar on the front? Did the PO leave a chunk of cardboard between the condensor and rad?

Wrong fan pitch? IE a reverse rotation fan blade stuck on the water pump? FWIW I ran my RR without a fan for a short time when I had a long freeway commute to work, it stayed cool on the road but would warm up in traffic.

I din't recall if you changed the water pump but I have seen pumps with the impeller rusted off. Loose impeller is a possibility but I would think that would make noise.

How neglected was the cooling system when you got the truck? Rusty mud or somwthing resembling water?