The factory washers are excellent, keep using them. What keeps the factory body bolts exceptionally tight, originally, is the corrosion that occurs in the threads of the bolts, and bushings.
The factory body bushings are in two pieces, the opposite end from the bolt head has threads built into it. If those threads are corroded too badly, or aren't lubricated when reassembled, then the part will either strip from corrosion, or be unuseable next time.
I don't know what body lift kits do with the factory parts. If you use the factory pieces, add antiseize to them, work the bolts on and off to evenly spread it. That will keep them from further corroding.
If you don't feel confident in the holding power still, then add a drop or two of thread locker. The thread locking compounds do there job by plugging up the gap between the threads, just like corrosion. The difference is that the thread locker will fall apart when you apply loosening torque. Any large amount of corrosion will do it's very best to destroy both the bolt, and the threads in that special body mount(special order, if you are lucky). Regards,
