I have a 22 inch radiator in my Coronet but it does have a shroud. I'm using a clutch fan setup and a mechanical water pump. The 512 inch stroker that I was running last year would cruise all day at 170 to 180 degrees with a 180 thermostat. Since I'm running a Sniper system I programmed in 45 degrees of ignition timing when cruising down the freeway and I know the extra timing really helped.

You could have multiple issues so if it was me I'd pull the cooling system apart first and double check everything. Have the radiator flushed out, check the lower hose to make sure it isn't sucking closed at speed. Put a shroud on there. Make sure your blade is in the correct position, make sure the belt is tight. Pull the water pump and verify how many vanes it has as well as the clearance to the housing, pull the thermostat and make sure it is working correctly. Lots of things can go wrong with a cooling system........