I can only speak for the P15K Deluxe:
Measured straight down from the front most point of my P15K Deluxe OEM bumper back to the middle of the contact patch of the rear tire, it's 8' 0" exactly. To get past the contact patch, it's between 8' 2" and 8' 3". That means you would have 3" to 4" to split between backing the OEM bumper away from the truck cab (or your forward most point of your 8' 6" flatbed) and the rear most edge of the flatbed. If you have extra room between the front of the flatbed and the cab of the truck, that would help.