I too use Stant Superstats and drill four 1/8 holes to aid in flow.

If it were mine and the tubes in the radiator showed any gunk I'd take it to a good shop and have it rodded out.

When I was having overheating issues years ago with my Dart I tried water wetter and purple ice. They made zero difference. It was running a new replacement 24" radiator, 160° thermostat, black magic electric fan and a belt driven water pump. I replaced the copper with a 24" cross flow aluminum radiator and had to buy another fan because there wasn't enough room for the one I had, it still ran hot so I put a new Flow Kooler water pump on, no help. I was about to pull the motor when someone here suggest that I try a MP viscous fan. A friend of mine had a nearly new kit so I bought it, then cut and sectioned a 26" shroud to fit the 22" aluminum radiator. All my over heating issues went away, I'm satisfied that if I had done this in the beginning there would've never been issues. I still run this set up on my 434" small block and on the hottest days I've never seen it over 185°, even at the drag strip.