All of that said above and get a non contact thermometer ( HF has them ) and verify that the engine / radiator is getting as hot as the gauge shows, it could be a gauge - wiring issue.
When the car sat for the 20 years or so, did it have coolant in it? If so the radiator could be plugged and rotted internally. The thermostat may be blocked with debris that broke loose and contaminated the new coolant. BTW, a 160 F thermostat would not be my choice, these older engines do well with some heat in them. I would go with a 195 or at least a 180.
And on your original question, .060 out of the bores on a slant is nothing to worry about. Folks have taken them .100 over. I have a slant in my 68 Barracuda that is .080 over and it does not have over heating issues.
