i have one on my Olds; dont expect a huge change, but i have noticed better fuel economy. i did mine mostly for clearance purposes, but a little extra umph is to be had.
http://s831.photobucket.com/albums/...amp;current=0202101545a.jpg&newest=1
you can kind of see it here. this was the moch up, you can see it way better
http://s831.photobucket.com/albums/...&current=0705090835.jpg&newest=1
its actually 4 inches; 1 inch spread bore to square bore, 2 inch square bore spacer, and another square bore to spread bore. i made custom ground phenolic inserts for the adapters.

66' Chevy C-30 Rat