If it were me I'd look to use something like a RCBS 45-500 FN without a GC and get that bullet coated if using a suppressor. Cast bullets have a lot less friction compared to jacketed bullets.
At 1000 fps it is not going to be loud, though a longer barrel would help if not using a suppressor. That particular bullet is designed to fit and function in the Marlin but importantly it takes up a lot of internal space, making reliable sub sonic loads easier to achieve.
This +1; you'll have a much easier and safer time working out a good load with a cast bullet, especially if it's of appropriate hardness and not the excessively hard commercial alloys that are so common.
Also, this is very important - when developing subsonic reduced loads, work DOWN in powder charge, not up! You aren't worried about an upper pressure limit, you're trying to stay above the lower limit of sticking a bullet in the bore, so you work down towards that instead of guessing at a low charge when you don't know where the sticking point is. That happens at much higher velocity for jacketed bullets than for cast. As velocity decreases, you'll notice velocity variation of the jacketed bullets will go nuts shortly before you stick bullets. Cast bullets should be fine down to 600 fps or less, but your jacketed bullet may stick as high as 800-900 fps in that big 45/70 case.
It might be overly optimistic to expect "whisper quiet" from a 16" 45/70; that's a lot harder to do in the big bore than in medium/small bores. 30+ grain powder charges won't help you in that goal either; that's why most experienced suppressed/subsonic handloaders look to small charges of fast pistol powders.
