Here is a conversion I did for my COL Straken.  It was actually very simple.  It utilized the Praetorian Lieutenant, a space marine plasma pistol, and the old Straken model.  Here are the original models before any work was done.

 Next I cut the arm off of the Lieutenant right at the epaulet. I also removed the bolt pistol which surprisingly left the holster details.  I trimmed the hand from the plasma pistol and glued it on.  It was a good fit.

Finally, I cut the Straken model's shotgun arm just below the elbow.  I actually started above the elbow but found that it was too much.  I slowly started removing metal until I found the right fit.  Here is a front and back view of the final model.


I think it is a convincing substitute and can't wait to get it painted up.