The place that installed mine fabbed 4 big brackets, bolted those to the truck frame and bolted the insert to those. Probably overkill but definitely good to go.
No way would I put an 8 foot insert in a 6.5 foot bed. Overloading the rear axle is a big enough problem as it is, you will compound it greatly by adding all that loaded weight behind the axle. I'd either get a 6.5 foot insert or better yet a pickup with a useful bed- a full 8 feet. Never made any sense to me to buy a work truck with a dinky little bed.