I agree you're better off trading to a different motor, than trying to build-up your 5.4L to tow that much weight.
For 7-10k towing, the V10 is perfectly adequate assuming proper gearing.
My (V10 with 4.56 gears) combined weight is typically 22,000 pounds. At 17,000 pounds scale-weight I can walk away from my dad's 5.4L van weighing about 8,000 pounds. His is stock with 3.55 gears.
2000 E450 dually V10 wagon