LQ4's in Trans Am's is race car shit. It will swap right in and bolt up no problem. You have to run your T/A intake, oil pan and accessories. Harness and computer is all the same.
A couple things is you have to drill and tap an extra hole in the block to mount the PS pump or alternator, I forget which. Also If it is a 99 or 00 LQ4 with the iron heads the crank is 1/4' longer and won't mate to your trans.
You will need an f body harness. If it's a 98 T/A I would look for a 99+ PCM and harness. They can be found on tech pretty cheap. Unless you want to cut up the LQ harness and run it as a stand alone. The f body harness will have all the provisions for the TCM and BCM.
I just noticed you said 2003 LQ4 so you're good on everything lining up and plugging in.