I have a Savage 50 cal and 2 custom Savage with 45 cal barrels. I have been shooting smokeless for quite a few years.
You do not need to fill out paper work for the government for any Savage muzzleloader except the very first production run guns that had a different bolt configuration. You don't want those first guns anyway.
Yes a TC 45/70 can be converted and yes conversions are being done on Savage center fire platforms also along with a bunch of Remington 700 MZ's. You will see a bunch of guys have gone to sabotless loads, .40, .416, etc. The .416 crowd are shooting heavy higher BC bullets with stout powder loads and stretching these guns way out past 300 yds with sub MOA accuracy. However that comes with a lot of recoil and muzzle breaks. The .40 sabotless is popular because it is mild recoil and easy to set up the two most common bullets, Barnes 195 gr and the SST 200 gr. to fit the bore
I shoot aftermarket 45 cal barrels, shoot 195 gr Barnes with a sabot, 2650 FPS, extremely accurate, low recoil, point and shoot to 200 yds with ease. These do all I need so I have not ventured into the sabotless world but if I did it would be a .40 cal. Send me a message and a phone number and I can walk through the basics because there is a lot of information to work through.
The pioneers of the smokeless MZ's are at the below link. There are 5-6 guys on there that build or assist in the builds, a few of them in Ohio. Everything you need to know or need to obtain is there. Be prepared to do a bunch of reading as there is a wealth of knowledge to be had.
I do have a 50 cal/SS/laminate stock Savage that I will probably sell soon, but if you have checked pricing they are not an inexpensive gun.
http://dougsmessageboards.proboards.com/board/23/savage-smokeless-muzzleloading