changing the stat will do nothing except delay the water from circulating
If the car was doing the same before the new rad....you either have an internal issue like bad waterpump...head gasket....or both setups were not installed properly.
I use a 6 blade steel fan with clutch,shroud and the fan blades are halt in/out of shroud. A good waterpump and this setup should cool that motor easily. 50/50 water..antifreeze also. I have seen high volume pumps push water too fast thru the rad...but even with a fast pump, the car should be cool running at highway speeds. Most issues are in idle/traffic.
If the car is doing ok down the road...and not in traffic...it is not getting enough air flow thru the rad at traffic speeds.