Either will work. 3031, being faster, will require a lighter charge and won't get as high velocity given the same pressure. 4350 will take more powder but can generate a bit more velocity.

If you're not into high velocity, you can save a bit of money with the faster powder. Load density doesn't have a significant effect on accuracy in an iron-sighted battle rifle. Let the guys with heavy-barrel scoped cannons worry about that.

Maximum load data from an IMR booklet -

.30-40 KRAG
WIN. CASE; WIN. 8 1/2-120 PR
REM. 180 GR. SPCL
.308" DIA.; 25.5" BBL.; 3.00" C.O.L.
Powder  Grs. M.Vel C.U.P.
IMR 3031 38.0 2375 39300
IMR 4350 46.0 2445 38700 [compressed]