If she ain't overheating in traffic then it surely points to slippage somewhere. Unless the Z is not channeling air through the rad when you are moving, but that I have not heard of before.

Meanwhile my '97 454 truck runs at 200-210 normally. Albeit I'm in Austin and it's normally ~100 F this time of year.

It is common for the V belt to not fit the aftermarket pully grooves right. Make sure it is wide enough and not bottoming out or it will slip.

What coolant mixture are you running? For hotter blooded apps I rec 65% water and 35% glycol. Lastly you did not mention what radiator you are using?

Regards, Kitz