Looks like the problem isn't seen interesting here, but you can try to split the body with a horizontal plane and then extrude the new bottom face to the wanted body height. I guess it works perfectly if the top edge is in the same horizontal plane. In my example it's only a partial solution.
I said "guess" because I do not have Fusion 360 and cannot be sure how it allows one to edit pieces which were originally triangle meshes.
An example
A plane is inserted to the highest possible elevation which does not clip the top surface bumps:
The body is split and the bottom part is removed:
The new bottom face is extruded downwards:
You can at first try to simply push the original bottom upwards to get the same as by splitting with a plane, but I could not use it. As said above, I do not use Fusion 360. In my program the original bottom was a combined surface which contained at least 100 triangles and many of them were very small. It was too difficult to select them all and to select nothing else at the same time. Splitting with a plane generated a single piece bottom.