I shoot a couple of different 45-70's, 2 rifles and 1 contender handgun. I think the speeds you're after are quite easily attainable with non gas checked bullets and very little leading. I would be very surprised if your chamber throat was too short for the 405 gr bullet. IMO that is the "standard" projectile that chamber should be cut for. I shoot 350, 390, 405 and 535 gr postell (bore riding nose) bullets out of my contender handgun bbl. They all chamber with no problems. I mostly shoot real blackpowder out of all my 45-70's now, including the contender handgun. It's very accurate and a real "blast" to shoot.
As long as your bullets fit your bore diameter, and aren't too hard, you should be fine. I assume you are loading commercial hard cast bullets? Those are borderline too hard for the speeds you're after, but they will work. Or are you casting your own bullets? I cast my own usually and unless I'm after some real high speed bullets (all things relative to the 45-70 of course), I usually cast them softer than commercial bullets. My blackpowder bullets are very soft. Too hard will lead as much or more than too soft at your target speeds (1400-1500).
You can test fit a seated bullet in the barrel, without having the bbl on the frame. It should just fall in and seat til the rim goes flush to the breach. If it doesn't, just slowly turn down the seating die in small increments until it does. I very much doubt you will have any troubles at all. Also, you will need to slightly bell the case mouth before seating the bullet in the case to prevent lead shaving. I normally bell enough to get the entire heel of the bullet (up to the first lube ring) into the case before the bullet meets resistance of the case walls. I'm also shooting soft bullets. You will then need to "crimp" the case down again to chamber. Since it's a single shot, I only "crimp" the case enough to just barely flatten the mouth back down to even with the case body. You do not need to crimp the bullets, and you can also ignore the crimp groove in the bullet for the same reason.
Feel free to PM me if you want as I do have some smokeless load data that I think would work quite well for you.