This is where I can not get a difinitive answer. If your Encore/G2 Contender did not leave the factory as a pistol and was originally registered as a pistol, it is illegal to make it one, don't matter whether it is done in the privacy of your house or not. If you put a pistol grip and barrel that makes it shorter than 26" overall, it is illegal. A NEF Huntsman would fall under the same rules as an Encore/G2 Rifle, they can only legally be rifles, and cannot legally be made into pistols, even though another that looks just like it that started life as a pistol can. I know there is no physical difference, but that is the way I have came to understand the law.
On the other hand, the NEF Sidekick is not considered a firearm, it is considered an antique (muzzleloader only), and requires no FFL paperwork. From what I am gathering, even though this Sidekick started life as a rifle, since it is a muzzleloader only, it is quite possible to have it made into a pistol.
There is a difference, and nothing actually spells it out for the non 4473 muzzleloaders.