For your Rune Priest I would suggest removing his left arm up to his elbow guard. Leave the elbow guard and the ribbing (where the arm would bend) on the model.
Then cut the lightning claw so that the large "fist" part is separate. File where they go together smooth and pin them together.
For his right arm I'm not sure how you could do it without removing the whole arm. I know a complicated way to do it though. You might try cutting/filing his arm off so you have a filled in shoulder pad. Then take the straight plastic right arm from the space marine captain box, cut the fore arm off like you did for the left arm and the part you would normally glue the shoulder pad onto, then glue that on the rune priest's shoulder where you filed the arm away. Finally glue the "fist" part of the lightning claw to the arm.
I'm a visual person so I don't know if that explanation makes any sense to you. Maybe someone else knows an easier way to do it too.
For the wolf scout you could do it by filing the plasma gun away if you want it to look proper. It will take a long way that way, but then you can just plop the meltagun into his hands. If you aren't concerned that it looks like a true meltagun you could cover the vent part of the plasma gun with green stuff, then replace the front of the plasma gun with the front of the meltagun and slice of the egg think off of the meltagun and glue it to your model.
Hope that helps, or at least makes sense.
P.s.The wolf scout with meltagun is still available too.