Could timing have anything to do with it? I had it timed but i don't know if that would effect the cooling system.
When you replaced the tstat did you keep the bleeder ball in place? You may have air trapped in the cooling system or that radiator may just be too small. I had to run my truck for a few days before all the air made it out of the cooling system after I rebuilt it, after that it stayed cool. Do you have a coolant filter? X2 on checking the fan clutch as well.
220 on the aftermarket gauge