Car overheats during city driving?

Heating up under cruise (load) means that capacity isn't enough. If it was another problem, it would overheat at idle, where the load is much less and the fan(s) work better

I can't imagine you haven't, but is the PROM properly tuned? I suspect you already know this, but if not, the ECM must be tuned to keep from running lean.

Outside of that, since it occurred with the heads, there is either a problem there (unlikely, you'd see other issues if it was something with an improper headgasket or cooling passage lineup) or the cooling system simply isn't adequate.

Have you gone with a higher end water pump? Moving coolant faster is never bad (ignore those who don't understand physics) but factory/aftermarket pumps aren't normally very good at forcing fluid through the system...they work, but they have no "excess capacity". With the Corvette, you are limited in area for the radiator, but on other GM engines, even when they couldn't actually pull air through the radiator, they added capacity in the form of coolant...the radiators are wider than the core support opening, but adds a gallon or more of coolant.

I have one of the Corvette aluminum radiators for a project, and I know its fluid capacity is compared to larger (such as trucks) radiators...it's about half the coolant. So you are at a significant disadvantage. I've seen people dump huge radiators in a vehicle that "overheats" with no success, so I can't simply say go buy the biggest/thickest AL radiator you can fit, because it may or may not be the sole solution. A high volume water pump won't hurt anything (Edlebrock makes them, stewarts makes them, probably even summit has their own branded ones) and an aluminum body will also help shed heat...might be miniscule, but with heating problems, every little bit helps.

You can also pull the thermostat and run it, see if the same thing happens. That will prove/disprove the thermostat as the cause. If it overheats without the thermostat (it should, since the thermostat should be full open by 230*) you know the t-stat isn't your issue.

Edit: People...the cooling system is pressurized. Do air hoses collapse when you flow air through them? Hoses don't collapse on modern (late 60's up for GM) cooling systems. That spring isn't there because of what you THINK it does.

Is the water pump good? Did the thermostat go bad already? Otherwise you are looking at a cracked block, a cracked head, or a bad head gasket.

You didn't put in pure antifreeze, did you? The less glycol the better the cooling. Straight antifreeze causes overheating . Evans waterless coolant is the best for an expensive cooling system. Check your ignition timing, retarded timing causes overheating.

Check the fluids of your car. This article may give you some insights.

http://goarticles.com/article/8653498

Check your radiator hoses as they may be collapsing in on them self thus cutting off the flow of coolant

are your fans working?