I have been fighting this for a while and I'm running out of ideas. The truck has a 1958 Chevy 261 inline 6 with a 200 4r trans. The radiator is the original 51 GMC that I had cleaned out. 7# cap. I also had the heater core cleaned out at the same time. I'm running a 235 848 head that was completely rebuilt the same time the motor was rebuilt - I have 1000 miles on this motor. They drilled the head for the steam holes too. The truck will stay cool when the air temperature is cool. But when it gets warmer, say 70 to 80 degress it boils over after about 5 to 10 miles. I checked the t-stat and even put a new one in it. It's a 180 degree. I put on a new water pump with no change. I pulled all the hoses off to check for blockage. I replaced the lower radiator hose with a heavy duty one, so it's not collapsing. I ran water through the radiator to make sure it wasn't blocked. My fan is the stock fan on the 261 and it's about 3/4" away from the radiator. I was running a smaller 235 carb and it was running lean. Now I have the correct 261 carb (thanks Poverty Flats) and it seems to be much better. I've adjusted the timing with no results. I don't want to run a fan shroud and I don't want to run an electric fan. I really don't want an aluminum radiator either but I'm totally out of options. I'm taking the truck to my mechanic in the morning but I'm afraid he's gonna tell me to get a new rad and shroud and... Anybody else got any ideas? They used to flog the heck out these truck motors back in the day. Why won't mine stay cool? Thanks for any help and sorry about the random thoughts.