When I did my S-10 I bent tabs up from the existing inner bed and spot welding to them. If you have any inner fender you mights could do this. Or, you could just spot weld it to the body like mentioned earier if it will be getting new paint. I used 3M body panel adhesive to seal the cracks. Then shot rubber undercoating over it all.
Honestly, that was one of the hardest parts of my project. I dreaded it untill it was done, and still would dread it again.