radiator??

My 408 runs a little hotter than I like at all speeds. It only has about 2500 miles on it and I had it hot tanked twice and recently flushed it twice and didn't get much out so I'm reasonably sure it's real clean inside. I had a 6 blade water pump on it and thought maybe it just wasn't flowing enough because I have March power and amp serpentine pulleys that slow the pump and alternator down a little so I just installed a high volume 8 blade pump and it made very little difference. I'm sure it's not running lean, got a A/F gauge in it and have read the plugs and if anything it's a tad rich, but not much. Timing is dialed in good too so I'm sure it's not the tune. First I had a 6 blade flex fan with a shroud properly mounted and in my quest to find lower temps I have tried a 16" electric puller fan that made absolutely no difference except tax the electrical system more. It'll idle all day without going over 185 but as soon as I get on the road it goes up to 210~215 pretty fast. I've tried 2 high flow 180 thermostats with the exact same result. I'd like to get it down to 200 or below if possible. I used a laser temp gun and it shows the water coming out of the radiator is less than 10 degrees cooler than what's going in. IMO the radiator isn't doing it's job well enough because it has plenty of air flow as you can feel it sucking all the way to the front of the grills. It's an aftermarket aluminum radiator with a core size of 19.25" x 18.5". It kept my old loose goose 360 (approx. 350 hp) cool but not the 408 (approx. 450-460 hp). The core opening is 3" wider than the current radiators core so I can go bigger. When looking at what Summit had to offer I see they have what they call a triple pass radiator that's supposed to be better than a std. single pass but I've never heard of them or saw them used. Does anyone have any input on them? What's everybody else running with their 450'ish hp engines? Also can anyone tell me why a cross flow radiator is supposed to be better than the old style down flow? For the last 30 yrs. everyone has said cross flow is better but no one I know knows why.