My older brothers second gen Camaro. He took a 455 Rocket from a 1970 Oldsmobile and rebuilt it with a big cam and all the goodies. It had long tubes headers and "zoomies" which were 4 pipes out each side of the car. The sound was absolutely ridiculous and the whole car would rock side to side...
The snub idler is probably OK as long as the belt isn't hanging over the edge. My Metco idler looks like that with the belt right at the rear edge.
As for the alternator alignment, I'd suggest picking up a Gates DriveAlign laser alignment tool, part# 91075. Otherwise you're just guessing at...