There are a number of people here who are much better informed on this sort of thing, but since there has not been a reply yet I'll try to get things started.

It is an 8" shell of some type, but was it a mortar shell or form a seacost gun (8" Columbiad or Howitzer.)
Where in particular was the shell found? Can you tie it to any known actions on that section of the Arkansas?

Does the shell have tong holes? (I suspect guns in this caliber also had them, so this might not help differentiate at all.) Does the fuse opening show signs of having been threaded or does it appear to have been smooth? (Again, I'm not sure if they differed in this caliber.) Is there any indication that the projectile contained balls? (Making it a spherical case shot.) Does it have a spherical cavity or a polygonal one? (Smaller caliber CSA shells sometimes had polygonal cavities don't know if the bigger ones were cast that way.) I've got one outline diagram of an 8" cannon shell vs. 8" mortar that appears to show a thicker wall for the cannon shell (perhaps necessary for the heavier propellant charge), so if you can estimate the wall thickness it might provide the answer. The difference in thickness appears to be about 1.5" for the Columbiad vs. 1" for the mortar--assuming the picture is representative of either/both. It also shows rather prominent inner lip ring for the fuse entrance extending into the cannon shell and none for the mortar.

I'm not sure that these are appropriate questions for this size shell but someone else probably will.