I doubt the seats use the same bolt pattern. I have an OBS '92 Blazer with seats from an '08 duramax and it was definitely not bolt in. Holes were drilled into the floorboard with backing washers/plates. It sites cockeyed as well, in that there is not enough room on the passenger side to grab the seat back adjuster. There is also clearance issues with some of the interior plastics around the original seat belt area. I run a stripped interior from the doors back.
I probably wouldn't have went this route myself, but it was already done when I bought the truck. I doubt your 99 with 03's would be any different.