Go 5.3L and never look back, also I would not recommend the AX15 dehind the 5.3L, it will live a very short life. for ease of installation, look for a 700R4. if you want a little better milage and the hassel is screwing with computers, 4l60e, 4l65e etc.. If you MUST have a manual, look for the NV4500, it will cost you up front, but you will have a bullet proof 3 speed with an over drive plus a granny low gear (5 gears total)
The 5.3 also shares many components with an LS series motor because... wait for it... it is a LS series motor.
With a simple cam swap and some fuel delivery tweeks in the ECM, 5.3's can make well north of 450 hp. There was an article in Hotrod not too long ago where they put twin turbo's on what they thought was a stock 5.3, made well over 1000 hp at something like 20# of boost. they made over 50 dyno pulls, decided to pull the oil pan and inspect the bottom end, they ran the castings and revealed that they had a 4.8L these motors are stupid strong and reliable.