Nice deer... good shooting. I imagine if the accuracy is sufficient, you should be good out to 100yds with it.
I cannot speak with authority on the National Firearms Act, but I imagine there are those on this board that know about it. I have heard that if you buy a T/C Contender as a "carbine", it is supposed to be illegal to put a pistol grip and barrel on it. I don't know how the "26 inch" rule comes in to play... or if it is, indeed, a rule.
I would email the ATF, or whatever they call themselves now, and try to get an answer from them in writing. I don't think I'd mention that I'd already DONE the conversion, though.