360 running hot

I rebuilt a 90 360, new rods, KB107 pistons and old crank. The crank journals showed no wear. It has fine oil pressure, runs great and will idle for hours and not overheat. It has about 200 miles on it plus more then a few hours idling. The heads are new magnums, the block was well cleaned at the machine shop. The cam is a small roller, 209 at @.050 and .459 lift. The timing is 16* at idle with the vacuum to the distributor disconnected and 32* at 3,000. It runs strong but on a long drive today it got hot. The radiator is out of a 73 Valiant 318 with air and it has the 8 ? blade water pump. I'm using the shroud from the Valiant. I'm hoping it's not too small of a radiator, maybe less timing? It seems to heat up while driving around 3,000 rpm. With the radiator cap off I can see good water circulation.