Since it's a Tromix upper (good call, by the way, guaranteed to be on-spec and match the chamber gauge which you will absolutely LOVE), the Lee 340, 400, and 500 RFN moulds will all fit, but you might have to lap them or powder coat them to make them a full .4585" and for subsonics in a carbine I'd suggest sticking with the heaviest bullet you can get. If you powder coat, check out that bullet FLT designed, it's an NOE and he's having really good luck with it in his rifle.
The problem with a lot of 45/70 bullets is when they get up around 450 grains, the noses are too long for the Socom. The 45/70 single-shot rifles don't have an OAL restriction, but do have a black powder capacity limitation, so with heavy bullets, traditionally the extra metal is put on the nose end rather than crowd powder space. For example, the Lyman 535 Postell and 505 hemispherical nose bullets will not work well in the socom because the front driving band will be halfway down in the neck.
Using GI or Lancer magazines, the bullet can stick out of the case an absolute maximum .690". The Lee 500 RF gas check bullet must be seated just a tiny bit deeper than will crimp in the crimp groove, just about .015" or so, and at that depth the front driving band is just about to "kiss" the throat leade.
OK, for the loading, you're probably going to be stuck using Reloder 7 for subsonics, since that barrel will have a carbine-length gas system. Walk that line between reliable cycling and cracking the sound barrier. Hopefully a faster powder will work, but pressure drops off fast and velocity also builds very quickly in a 16" barrel. You'll just have to play with it and find out.
You'll need a neck expanding die, a real one, not just a belling die. Unless your bullets are over 15 bhn, you're going to have trouble with the Lee expander not making the neck large enough to prevent the bullet being swaged. It's perfect for jax though. I made a custom spud for mine (I forget the size at the moment), just for softer cast bullets.
Some people cuss the Lee dies and everyone seems to recommend Hornady, but my Lee set works fine. Full cam-over makes the brass .001 below flush of the case gauge, right where Tony wants it. Don't use too much crimp, just barely turn the mouth past straight. Do chamfer your new brass on the inside before loading the first time. Check EVERY cartridge in the gauge before trying to shoot it. Don't drop a round in the chamber and slam the bolt home on it, the thin pistol primers can slam-fire, and DO NOT use Federal primers, they are too sensitive. Get a Caldwell rail-mount brass catcher if you don't already have one, brass disappears and you'll cry a lot.
When re-sizing fired brass, check headspace. Fired brass likely will likely stick out of the case gauge .020" or more, don't be alarmed, it extracts under pressure and the shoulder can move forward a lot and that doesn't mean the headspace is excessive. Probably less of a factor in a carbine system than my pistol system, though. Brass will shorten ten thousandths after firing, and probably stay short when resized, so watch your crimp die settings very closely. For this reason, the Lee factory crimp die is highly recommended, you can use it to remove the mouth flare after seating and there's a good bit of length tolerance there so you aren't having to constantly re-adjust your seating die to deal with the moving target of case length.
I'm dumping all this info on you because I'm taking a vacation for a couple weeks starting tomorrow and am not sure I'll have good internet where I'm going. Should have, but who knows.