Hi Tamb and
to the forum.
The GM 26" MPII, MPIII, MP4 and MP5 wheels all use the same spokes:
The Sapim Leader 13G spokes should fit your original nipples, but the Sapim Strong 13/14G butted spokes would require the smaller 14G nipples to suit the smaller thread:
As you are only planning to replace two spokes, I suggest that you use the
13G (non-butted) thicker diameter spokes which are more comparable to your existing spokes and should use the same nipples.
However, if you were rebuilding the wheel using a standard 36 hole bicycle rim using all new spokes, I would recommend using the 13/14G butted spokes with 14G nipples.
Please note that fitting a different rim may require different length spokes if the rim depth is different.
Alan