mayito

I have a 2003 V6 silverado that i recently put a 5.3 V8 out of a 2005 truck. Everything was changed with the wiring. All of the 2005 wirings went into my truck. When I went to go pick it up, everything was fine until i got home and then all of the guages turned off and then the truck turned off. I changed the battery and it started up again up until and hour driving then the voltage went way down and finally my truck turned off again. My mechanic changed the alternator, and some module and it still does the same thing. All the grounds were correct but he can't find the problem. The battery i put was brand new. Any help would be appreciated. With out my truck