SLT-6 is 0.060 Over - Runs Hot...Why?????

I don't know what's makin it run hot but it AIN'T the .060" over bore. As John said, slant sixes have been taken out to .100 and beyond. They AIN'T a thin casting. From stone cold, start it up without the radiator cap. If the radiator is SLAP full, drain some out until the level is about 1" below the top of the tank. Let it run until you see and know the thermostat is open. You'll know this by the coolant getting hot and starting to circulate. Once that happens, reach in and goose the gas a few times. If it gushes coolant out of the radiator when you goose it, the radiator is stopped up and the coolant has no place to go.

It could also be a stuck thermostat. If the coolant in the radiator never heats up and it does not begin to circulate, the thermostat is likely stuck closed. Do you have a thermal temperature gun? Best 20 bucks you'll ever spend. Maybe 10 if Harbor Freight is runnin a sale. You can determine a lot of things up to and including verifying a stuck thermostat, measuring incoming and outgoing coolant temp from the radiator and other nifty things. Cheap as they are, this is a tool you should already have, especially trying to troubleshoot an over heating issue.

"I had the radiator re-cored" is no reason to overlook the radiator as a suspect. Remember, "new" and "rebuilt" do not mean "GOOD".

Oh and get that STUPID 160* thermostat outta there and install the correct 195*. The thermostat has ZERO to do with how hot an engine runs. It needs the correct thermostat OR no cooler than a 180*.