PK numbers
Do not confuse horsepower and torque.
You may be able to get circa 1985 360 horsepower out of your slanty, at some higher rpm. But you can never get 360 torque out of it unless you supercharge it.
And to tow 3000 pounds, with 3.21 gears, you are gonna need torque, cuz 65= about 2800rpm (10% slip) with 28"tall tires.
My guess is at 2800, it will take 150 footpounds..... or more. It's gonna take a lotta lotta throttle......
A loc-up will reduce your rpm to 2500, so Ima thinking; forget 150 ftlbs.....
Lemmee put it into perspective;
Suppose you get your 225 up to 140 hp at 4600rpm. that would be 160 ftpounds......... at 4600 rpm. How much will that be at 2800rpm?
IDK.
Enough to tow a 3000# pop-up , and propel nearly 4000pounds of truck, built lake a brick with a huge frontal cross-section. Why sure, but will it do 65mph?
IDK.
But even if it does, it might have to be floored.