A friend of mine recently got an old 45-70 from his dad which was originally owned by his grandfather. Serial number is 41xxx. A quick online search has it manufactured in 1890. It is stamped 45 Gov't. I wonder if you could help me with pressure limitations as well as how to take the gun apart to clean it. What model would this be?

Likely a model 1881, about the first successful repeating rifle to handle the 45/70. I believe the 81 was the only top eject gun marlin made, so if it's an open top that's what your friend has.
If it's in good enough shape to shoot I would definitely stick with trapdoor safe loads. While factory stuff would likely be safe I would probably cook up some mild loads using cast bullets and Trailboss.

rufous, can you give a little more info on the gun?
make, model, etc?
most current factory ammo for 45-70 is loaded with older guns in mind. there are still a lot of trapdoors around also 81's , 86's and h&r shikaris.

sorry . now I see it is a marlin.