I ran 2-1/2" all the way to the back of the truck, but ran the tail pipes on the outside of the frame since I am running a rear fuel tank.
I prefer the old school sound of Cherry Bombs, and with the long tail pipes they sound really good, IMHO.
More details of the install can be found here
http://67-72chevytrucks.com/vboard/s...=363509&page=6
and here is a video clip
Hope this helps!