As you research this...keep in mind that the rear seat in an '03 differs from the rear seat in the rest of the original interior model years. I don't know if it impacts the mounting locations or anything else that you'd be worried about; but figured it was worth a mention. I know they changed 'something.' See diagrams below.
One interesting thing...the 'riser' for the 04-07 H2s (PN 88941240), is listed as a cross platform part used on 3500s, Sierras, Silverados, Yukons, etc. So...if the bolt pattern didn't change on the deuce from 03 to 04 you might be able to pull a set of rear captains chairs out of a Denali or Escalade and have them bolt right up.
http://parts.nalleygmc.com/products/.../88941240.html
'03 diagram
'05-07 diagram
