I've just completed a .260 Rem build on a Mauser action with a 1:9" twist, 27" Shillen barrel. I'm planning on shooting the 123 gr SMK because I'm doubtful anything in the 130/140 gr weight class will properly stabilize.

I'm getting pretty confused on what powder to use with the SMK. On hand I have H4831, Varget and IMR4064(I'd really rather not go buy more powder if I don't have to). I'm reading that Varget is too fast and H4831 is too slow. Can anyone set me straight on how these powders will perform inside 500 yds and what would be the better of the three to use with the 123 gr SMK. Thanks for the help!!

i use 4831 with good results. otherwise try the 4064

Both the Varget and 4064 would be good. The 4831 is not really listed for the lighter bullets but is for the heavier ones. Touching on you comment about twist stabilizing the 140 grain and heavier bullets I can tell you that I also built a 260 on a Mauser action, but it's a Jap Ariska (which is a copy of the 1893 Mauser action). My barrel has a 9 twist also and I throated the chamber throat out so I could load 140 grain bullets out of the powder space of the case because the Arisaka action is plently long enough and they shoot really good. No signs of bullet tipping. In fact I have shot a cast bullet weighing 172 grains which is very long and it stabilized it just fine. Cast does act a little different then jacketed though so I wouldn't rely on it stabilizing a 160 grain bullet or heavier with a 9 twist. A 140 grain bullet is a piece of cake in that 9 twist.

So just choose which powder you want to use between the two you have that are suitable. Hodgon's online data has loads for both those.
Here's the link:

http://data.hodgdon.com/main_menu.asp

Mine really likes 4064 for 100-129 grain bullets. All of my 6.5s from 260 to my 264WM shoot excellently with a 1:9 twist. They all have it except my 6.5x284 which is 1:8 for the heavier match bullets.

with the powders you named for the light side 4064.. H4831 is good for the heavy side 140s+ i shoot 120s and some 129s i was loading RL-15 but now im loading H-4350, but yes, i have had good results with 4064