I think I am running that same thermostat in my stroker motor in my '77 GMC, and that thing never turns on either of its fans. I think I just stuck in the cheapest thermostat I could find, so probably worth experimenting with a new one. That's cheap enough for me.
I would also flush the ENGINE as well. If that's not been done, it's worth doing. And if the engine is really dirty, go ahead and flush the radiator as well, since it is now contaminated. Get it all as clean as can be.
