50 out of 132 in the 24 Hours of Lemons: You are my hero.
I agree that a full size extra cooler will help, but given that you're essentially running a truly stock car into the ground on purpose, I don't know if I'd pay the extra $$ for a B&M. You might be able to find a lower priced alternative on Ebay or Amazon.
But, there's also the question of whether adding that much more surface area affects line pressure and changes the shift characteristics (not a transmission expert, just spit-ballin possibilities)
Curious: What's the sustained range of the RPMs you're typically running? I ask because you write that you keep it in OD Off, and it's truly a testament to the 4.6's durability to beat it like that.
2005 Lincoln Town Car Signature Limited
5.4 liter throttle body; PI oil cooler, intake tube & air box; Mark VIII aluminum driveshaft; Trac-Lok limited slip; Richmond Gear axle shafts; J-Mod with Sonnax valves; Limo rear control arms and a great sounding stereo that won't ever automatically remember my Bluetooth settings
1999 Mercury Grand Marquis (gave that to youngest son, and he's now a converted true believer)