If it is old enough it pre-dates any Contenders sold as anything except pistols. I don't know what that date is. Perhaps one of the other readers of this list can help with that.
What you do to it has nothing to do with how the federal laws apply. It matters how TC registered it with the government when it was made. Rumor has it that all Contenders and Encores are registered as pistols, but I have never seen that officially from TC and I am not willing to bet my freedom on a rumor. There are also state laws that you need to investigate and conform to as well.
One thing for sure is that with a buttstock on a frame if you put a barrel less than 16 inches long on it becomes an illegal short barrelled rifle, and that will land you in the klink.
If you are interested in finding out more about this google this topic and see what you get. You can read lots of opinions both ways, and also the results of the court case and make up your mind.
I am no attorney and don't play one on TV so this is not a legal opinion. You need to decide what you want to do.