I don't think scope bases for the M1885 will properly fit an M78, and vice-versa. I've had both, and the octagon barrel contour on the M78's is much slimmer then the M1885's contour - unless your M78 is a .45-70.
If your M78 is a .45-70, you might have a shot - as the two contours are very similar, if not identical.
Burris says that their TU-85 base(s) fits octagon barreled M78's.