if you guys want to really talk advanced exhaust set up then 1 5/8" headers make more power then 1 7/8" headers on a n/a engine (been proven, i can find the artical if you want. h/c LS1 on an engine dyno) while 3" is bigger than you need untill you get to the 500rwhp mark there won't be any hp loss going that big.
if you do 3" then you can run that to the axle into the mufflers then drop it down to 2 1/2" out of the mufflers and over the axle. this will help the with the exhaust scavenging because the gasses have cooled down by the time they get to the mufflers and take up less space.