When you purchased the firearm, did the dealer check "Rifle" or "Pistol" on the Form 4473?
If he checked Rifle, I'd keep a rifle barrel and stock on it.
If he checked Pistol, put whatever you want on it, but make sure that you don't put a buttstock on the frame unless the barrel on it is 16+".
If he _did_ check Rifle and you want to use it as a pistol, I'll throw out a potential idea. NOTE: I am not a lawyer.
Go to the friendliest gunshop in your area, or a "$15 FFL Fee" kitchen-table dealer. Strip your Encore down to the frame, sell it to him for $10. Buy it back from him for $10, plus his FFL fee. Ensure that he checks "Pistol" on the form 4473 when you buy it. You would then be the legal owner of an Encore pistol, which may also be used with a buttstock and 16+" barrel.
You legal eagles out there: does this work? Or does that serial number need to be officially listed as "built as a pistol receiver" back at the T/C factory?
If this idea does not meet the legal requirements, please let me know and I'll edit my post to put a big "THIS IS NOT A LEGALLY VIABLE IDEA, OFFERED AS NEGATIVE EXAMPLE ONLY" into this post.
-MV