Well... the guns shouldn't be that difficult.
Considering that shardcarbines are basically poisoned flechette shot/sub-machine guns, if you wanted them to look organic and grown/grafted into the wracks I'd probably go with tyranid bits. I don't know what the specific ones I'm thinking of are but they're sort of like pyramid shaped barrels with many small holes aimed outward. Noone seems to use them and every tyranid player has a bucket full for some reason.
Alternatively you could use the pre-existing spinal armatures to hold a more traditional weapon, even aimed over the shoulder techmarine style or up over the head with a talos like tail. Either way they'd still have both arms free to explain their bucket load of attacks.
As for the over all feel - the newer haemies suspend and move themselves on those spinal tentacle things. If you were able to get your hands on some spine/whip things from the talos box you could have your wracks lifted off the ground in much the same way. This would bulk them out on the 40mm base and make them fairly serpentine.
If you can't get your hands on talos bits, spines are pretty easy to make using green stuff and a tentacle/roll tool. You just roll it like a normal tube, then go back and do a lighter roll in the perpendicular to give the discs depressions. You can find these tools for $15-25 online, or see if one of your gaming group has one you can borrow.