One word. GUT.
My LB7 started at 6250lbs with me in it.
It's been as light as 5380 with me in it without a roll bar, currently is 5550 +/- 50lbs.
There is a lot of weight everywhere in these trucks, finding it is the problem.
The ZF is how much weight compared to the Allison?
I "think" I had one of the lightest weighing 4wd single cabs we know of, but could be wrong.
As far as the spray, I spray on launch and spray through the gears, never had a problem.
Since it seems like you cant stay in 5th due to RPM I'd almost consider finding a 33" slick and removing the 4wd make it a 2wd and you will be able to run 1320' in 5th gear.
zf 4x4 trans 233 pounds zf 2x4 trans is 241 that's with fluid