So here is the conclusion to this thread story. Thread summary - I changed every component in the cooling system. Tried 3-core alum rad, hoses, two temp sensors, 2 gauges, 2 water pumps, 2 thermostats. Checked timing, carb mix screws, etc etc. A really annoying overheating problem all year here in Las Vegas.
Mishimoto's twin fans (2600 CFM total) couldn't keep my '70 C10 402 from overheating at slow speeds/idle.
https://www.mishimoto.com/chevrolet-...oud-67-72.html
Their fan controller did work well though.
https://www.mishimoto.com/adjustable...oller-kit.html
So, those of you who said I just needed more airflow were dead ... right!!
I just put in two Derale 14" fans (2100 CFM each, 4200 total) and wow, those little hurricanes have no problem keeping the engine cool. Finally!!
I also installed Derale's PWM fan controller which automatically adjusts fan speed to match temp I set and also has a soft start ability. And yes, they run after ignition off but only for 1-2 mins at very low RPM, no problem to the battery.
So worth the $150 more for the Derales fans and controller than what I paid to Mishimoto. Perhaps other Chevy big blocks cool down well with the Mishimoto fans, but mine sure didn't. I guess I now know why there are no Mishimoto reviews on Summit or Amazon for the fans I got . They don't do the job.
So I tip my favorite hat to all you who helped me on this post, great team effort (and sense of humor). I'm a happy C10 man.
http://derale.com/products/electric-...fitting-detail
http://derale.com/products/electric-...n-probe-detail