Reinforcing box for small crane

[1968 Pontiac]

I'm thinking about installing a small crane onto the bed of my 1992 K2500 short bed pick-up. The crane is rated to lift about a 1000Lbs. The crane comes with a couple of angles ~3/16x2x2 for under the box and a plate ~3/8x6x12 for on the top side. I'm thinking that isn't very much considering the load it can handle. I'm concerned I'm going to rip the box apart the first time I lift an engine into the box. Does anyone have any simple ideas to reinforce the box or should I just run some custom formed steel from the bed to the frame? Due to limited reach, I need to mount the crane near the back of the box. I was thinking in the area between the wheel and tailgate on one of the sides, but the only problem there is the leaf spring mount is in the way. Or should I just build something that connects to the trailer hitch? I have a 7000 Lb Reese hitch. Any ideas would be helpful. Thanks.

[webbch]

I'd recommend reading some users' experiences with these cranes. From what I've gathered, 1000 Lbs is EXTREMELY optimistic, especially if considering any of the offerings from Harbor Freight. The realistic rating is more like about 500 Lbs max. If you get one that has a swivel base, about 200-300 Lbs and it starts getting difficult to rotate the base. Anway, here's the link:
Truck Hoist/Crane

It looks like you could probably lift an engine into the bed of your truck, but it sounds iffy about whether you could do it with the transmission attached.