2003 SS 6.0 ls1 swap into 98 GMC Serria

So bought a whole 2003 Chevy SS truck and took motor out of it and transmission to set into my 1998 Seirra 4x4 I kept the transfer case off of the GMC but put motor and tranny into truck. I set the transmission and transfer case in truck while waiting for my dirty dingo brackets to ship in. Last night put motor in with arrows on brackets pointing to the front of the truck and slid motor all the way back. I am using the same stock tranny mount bracket that was on the GMC but with motor slid back and trany bracket I have a 3 inch gap between motor and tranny. What am I doing wrong.

Any progress yet?