Though I'm a "longer the better fan" myself, I rode short tracks for many years prior to there being such a thing as a "real" long track. They (the long tracks) really haven't been around
all that long. You can still get to where you want to go with it, you just have to be a bit more creative! I can ride my wifes RMK 700 with the 136 x 2" into
most of the same places on the mountain as my 159 x 2 1/4" RMK 900, with a little extra effort,
okay, okay alot of extra in some places.
To a point, much of your riding has to do with your riding ability rather than what you're on. I say "to a point" because I can ride my 700 and 900 into steep mountain country much further than I can the old short track Phazer or Skidoo I used to ride back in the late eighties. But then again, they were what they were and they are still fine sleds in their own realm, even though I'd be long tracking them now.
To make a long story short,
I'd rather be on the 153" but you should do just fine on your 136, given that you understand that it' not your 2" 153", especially once you get the feel of the sled. PS: If it was me I'd put on a longer deeper track and take the extra advantage of what your sled is really capable of, but that's only my two cents, like I said I'm all about long tracks.
Most of all, have fun! I'd love to take that ride out of West Yellowstone someday!