Overheating 360 new engine

Normally one running hot at rest is air flow. Could be you don't have enough fan, OR it could be that radiator. There've been a couple different studies from testing that found a two row with at least 1" tubes cools better than a three row. Something about the three row having the flues crammed close together keeps the heat from shedding properly due in part to blocked air flow. I'm no thermodynamics engineer, but that's what the word is. Generally speaking, you need a minimum of close to if not more than 4,000 CFM to cool a hot rod engine. So if your fans are not close to that, that could be an issue. Also, are you using a high flow thermostat? If not, I highly suggest one. From your description though, I would look at air flow first.