Read numbers in the base of the largest digit and write it using underscores
Pyth, 26

JscR).zjd*R\_jsiRKhseSsJJK


Demonstration.

JscR).z
     .z                       Take the input as a list of lines.
  cR)                         Remove all whitespace from each line.
 s                            Combine the lines.
J                             Save the result to J.
                              J is now the list of the string forms of the numbers.
       jd*R\_jsiRKhseSsJJK
                      sJ      Combine J into one string.
                    eS        Find the maximum of that string.
                   s          Convert it to an integer.
                  h           Add one.
                 K            Save to K. K is the base.
               iRK      J     Convert the strings in J to base K.
              s               Add them together.
             j           K    Convert the result back to base K, as a list of ints.
         *R\_                 Map each int in that list to that many underscores.
       jd                     Join on spaces and print.