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Having a look at the pieces, I think you'd need a bit of modeling to make it work. I don't have my camera charged at the moment, so bear with my descriptions.
The Dev. Backpack for a heavy bolter has about 2/3's of the ammo chain coming off of it, looping up to connect with the part of the ammo chain that's on the gun. Obviously, on the non-Dev model, there's no connecting ammo chain on the gun. So you could either extend the chain, or mask it. Doing the former would be easy if you had another backpack to get more of the ammo chain from, or amazing modeling skills to build such a piece. Another way to do it, without such ability or items, is to connect the pieces with a less sculpted bit of green stuff, and then cover the area with a green stuff cloth or robe. This might actually look pretty good. I hope this helps, it's what I could come up with off the top of my head.
You will need a bit of tweaking and a little GS to make it work... nothing so special for a man of your talent! Although I have to llok to my bits box for spare bits if you like... 8Y
Thanks to all the people showing faith in me
I sometimes feel that everyone will leave me and they will be on their own...