if your going to the LS6 heads from the 853's, then yes, you will have to convert to a 4, center bolt set up, which does include changing your coil packs over to the bracket mounted ones. Im not sure if the coils are the same, and its only the way they are mounted that is different, or if the coil packs are just different all together since yours mounts by each one being seperate, vs the 4 on a bracket. You can also get fancy and mount you coil packs somewhere else, via a relocation bracket, longer wires, etc,,all depends on the wallet and the plan. So, from what i think your wanting to know,,you will also need:
2 center bolt valve covers, with seals
the 8 bolts and their seals (should come with your covers)
2 coil pack mounting brackets (if your coils are the same, im not sure, but i think they are)