the serials numbers are the same on both. the only sifference i can see is one is just a little bit longer. i need to be sure about this cuz im selling the 8 and keeping the 6 for my rear pumps. btw they're rockford gears from 2000 that came with my pro hopper pro comp g force pumps. they look like marzochis, but are in fact rockfords.
the ones with the thinner center section are the #6s from what I remember,but dont trust my memory. Those looke like marz style pump heard to me Pro Hopper had 'G-force" heads that were just marzochi knockoffs. I'd personally take them apart to make sure,but just remember how they go back together if you dont already know what it looks like in there. by the way, how many bolts does each have going into the block?