Quick question about muffler!!

That is all up to you and how you want your exhaust setup... It won't affect anything, but more dependent on how you want to route your exhaust. If you want it in the same place center in/out would be fine and he could put in pipe to close the gap

 

That would be the common route yes... But if you look at your exhaust pipe coming down through the pickup, if the pipe comes straight back through and doesn't move left or right, center in/out will work just fine. Just look,you'll be able to see whether or not the pipe moves. If worse comes to worse, take a couple photos, one from one side of the muffler, one from the other, and one straight underneath and I will tell you what you need...