Am about to exercise my latest 40-40 Maynard 1873. Have a couple of thoughts:
I have cases from Rocky Mountain, now under new management.
I found that my old LEE squeeze priming tool fits those rims almost exactly if I take out the shell holder entirely. It's almost as if Lee measured it!!

I'll use 4198 0r some 4759 I still have from long ago and far away.
breech seat paper patch.
Use just enough powder to stay just below speed of sound, 100 f/s or so. Why? 1200 is just safe, and if the bullet transitions flow through the speed of sound, groups go up in size. freaky things happen, especially in wind. That's why the 1000 yarders (not bp) keep their rounds faster than sound at 1000 yd.
Remarks?
Don

I stick with black powder in those 1873's mostly because of the firing pin design. The firing pin tip is .150" diameter and it is held in by a a#3 cross screw which is .099" diameter. The screw is at the bottom of the firing pin and less than half protrudes into the firing pin channel. Back force on the firing pin acts as a cam to direct the force upward thereby tending to loosen the rivets holding the conversion plate in place. 
I know of one instance where the firing pin was blown out of the action and the conversion plate was blown off and neither was found. The shooter was using triple 7 in a 40-40.
As noted also the frames date from the Civil War and the iron is full of slag inclusions. Many times these can be seen on the outside but no one knows what is on the inside. I once repaired a flash channel in a Smith carbine that was made by Mass Arms, the same outfit that made Maynards. I found a slag pocket the size of a pencil eraser inside that Smith frame.

John (Deadeye Bly) clearly and succinctly stated what I have also come to conclude about the Maynard and whether I should use smokeless powder in same.  In the words from the Wizard of Oz, "... not nohow!"  You asked for remarks and them's mine!  BTW, I have a Model 1873 in 40-40 and shoot it with Goex black powder.  Cleanup is quick and easy, but needs to be done thoroughly and fairly soon after you finish shooting.

That's my story and I'm sticking to it! 

Froggie