408 cooling issues

If it keeps going up a lot in temp both at idle and at speed, I'd love to say it's the radiator ;) but it is not in this case.
New motors run hot for about 500-1000 miles
You sure the fans are pulling and not pushing?
Timing
make sure your thermostat is not stuck and your water pump is working properly. with the fill cap off, make sure you see fluid moving at a decent pace.