66 Dart, 408 runs hot. HELP PLEASE!

As to the bypass system;
It's primary function is to shorten the warm-up period, and secondary function is to prevent dead-heading the coolant circulation, during said warm-up period, when the stat is closed; of the stock lo-po iron headed smogger engine, with the factory WP and fast idle cam at 1200 to 1500rpm..................
Yeah, no, I didn't see that applying any more, even on my little HO367,lol. I installed a Hi-Flo stat,and restricted my bypass to prevent excessive re-circulation of hot coolant.
Once the stat is open,this restrictor forces more of the the hot water to exit the engine and head for the rad.
While the stat is closed, this does force the water to slow down and stay in the engine a little longer. I use a 190 stat to delay full circulation a little longer cuz those aluminum heads take a looooong time to warm up.
I run a lot of timing on the cold engine, instead of a choke. I pull the timing out as she warms up.
I run a Vcan off the spark-port; to augment my lazy mechanical curve at part throttle; which allows me to run 87E10 on 185psi, and more, cylinder pressure.
I run underdrive pulleys to help keep the belts from de-railing at 7000/7200 my chosen WOT shift rpm, with catastrophic results usually following.
Once up to temp,my system runs at 205*F...... ALL the time, with an ancient 26" factory rad from a 1973 Dart 318/AC car.
I'm at about 420hp, your results may vary.
Just offering my success story.

As for your engine; with that much power on tap, I would certainly give up a couple of hp and run a direct-drive 7 blade fan and an anti-cav plate on the HD pump. This will prove where your issue is.
I like the T-clutch from the early 2000s Ford Pick-ups so, if I had to, I would sacrifice rad thickness for rad area, to be able to run it. That thing has been stone-axe reliable for me since 2004.