For the guys that went out and dropped the 13K into an LS7 crate motor, you've realized by now that there's more to invest before it'll work.  I thought I'd be able to use the wiring harness with the engine I purchased with 12K miles...but nope...I gotta spend the extra bucks too. 

The stock LS7 harness, ECU, gas pedal and O2 sensors...and some other items all must be replaced unless you are someone who wants to spend the next 6 months trying to get it to work in the GTM.  Forget it and purchase the standalone GM Performance kit.  The kit sells for about $1200 but it's worth it.  This kit will run the engine and provides what you need for the GTM.  They are available at several shops on the internet to include Summit, Jegs and probably 20 others.  The kit part number for the LS7 is 19166567. Because the GTM intake is rotated 180 degrees, installation into the GTM will require rerouting of some wires. Follow instructions in the GTM assembly manual for installation of this harness in the GTM. GM Performance Parts Website:http://www.chevroletperformance.com/home.jsp  Click the "Shop" tab on the upper menu bar on their website.  Then click on "LS Controller Kits". Here is the installation manual -