Repositioning the arms of tactical marines is somewhat tricky since essentially you need to remodel the entire chestplate. If possible you might want to look into other space marines for that. The chaplain model for example is very usefull.
The technique I've used when converting space marines is something like the following:
1: cut off the shoulderpads.
2: carefully cut off the bolter
3: remove the arms - these can be thrown away.
4: Cut the model in half by the torso if you want to reposition that. Otherwise you can leave it.
5: re-attach the shoulder pads in the position you'd like; some trimming on the pads as well at the back of the marine may be needed.
6: make a new pair of arms from thin plastic rod. It is easier to glue on a long piece and then trim it down to size. Depending on your craftsmanship you can carve gloves, fingers etc. into the arm.
7: glue on the weapon. IN case of lightning claws I'd start with a very thin piece of plasticard then add 3 thin strips of plasticard to represent the claws. It may be less that what's on the original model, but in 6mm the art of suggestion is sometimes much more effective than exact detail.
I got some pics of converted space marines somewhere (my Dark Angles) but I haven't got time to find them right now. Remind me after the weekend and I'll see what I can dig up
