As above, a product called water wetter may also help - they basically increase the heat transferring ability of the coolant. Not a ton more, but a little more. With ambient temps in the 90 range and constant heavy throttle needed I don't think it's good, but I also might not call it a normal problem. Retarded timing adds heat, as does a lean mixture. Sounds like the tune is pretty close as it is which makes me think it might be something more... When the engine was rebuilt - how were the block's cooling passages cleaned? If it was only boiled there may still be rust buildup on the cylinder walls. If it's taking too much throttle to run I'd suggest a gearing change to give the engine more leverage.
