1991 F150 4x4 4.9 liter inline 6 cyl

I want to put a smallish camper on my truck and was thinking about installing a 5th leaf spring in the rear. I found a pair for $80 and thought this was the way to go. I recently had 10 6x6 PT landscaping timbers in the back and it was sagging a bit.

I welcome your thoughts and any tips or suggestions for installing would be greatly appreciated.

Thanks,
John