Yeah, you need that special U shaped mount, that slips over the stock lower mount. You probably have the eye mount. I think they make kits that'll fix the mount on the shock itself, but i'd try taking your shocks back to wherever you bought them and exchange them for the right pair. I'm pretty sure rancho has them for the stock gm application.
I think that stock shock will be done for in no time. It has 3" of extra extension on it already, and if your suspension droops, that shock will probably rip itself off the mounts. For the time being, i'd just run no shocks in the front, until you get the right ones.
Where did you buy the shocks from??
Nice truck by the way, it looks really good. I agree with 4.88s