If an engine has stock heads, a bigger bore, and a stock-ish exhaust, it'll overheat in traffic on an 85* day. I had just such an engine in the first two cars I owned. We don't have much stop-n-go traffic here, but it gets 95* and both 1776s overheated.
There are lots of things you can do to bring the temps down incrementally. Ceramic coating the muffler and installing a venturi ring is going to be good for about .0001% of what ails you.
By far, the most bang for your buck is going to come from 2 things:
1) a 96 plate (or better) remote oil cooler, with a thermostatically controlled fan and bypass.
2) a quality free-flowing 4/1 header exhaust.
Neither are cheap, but both together will fix your issue.