most of us don't use those conveyers we usually take them out and the only thing you'll see of them on the inside is the place where they are mounted leave an empty hole.
You are right about the M109 lower chassis, you'll have your hands full if you make an ammo rack ( the vehicle holds 96 rounds [ complete rounds , shell, fuse, powders {there are two charges in the long skinny cannisters called green bag and there is one in the short cannister called a white bag}, there are a few other types of propellant but there would be to much to go into.....)
what the cat does is called close but seperate this is for example take your gun they are firing at point A and the cat is just for example 20 meters or less off doing security then the gun needs ammo the cat will pull over and open the side hatch and have a few guys help out so it goes faster, they'll upload the gun with the new ammo by hand right from the cat then go back to pulling security.
here's a link to the page I mentioned before, I'll try to get some more pics up of them, I know the ones on here aren't all that great so.........
http://rongeorge.com/modules/Gallery/faasv