I usually don't recommend stainless bolts for anything (engineering geek reasons) but for my skids I also used 1" long 6mm flange bolts (well, 25mm long).
There is extra threaded bolt sticking in to the pipe - it's not seen so no issue.
But after being soaked in Tennessee swamp water and packed with that special Stinking creek mud (ever notice how it smells like Stinking creek when you pressure wash the vehicle a month later?) I don't want to bolts to rust on the back side of the plus nuts.
If they do rust on the back side you will never get them out - just throw the p500 away and get a Talon at that point.