2 options from tracks USA, probably the best ext kit out there!
http://www.tracksusa.com/Rush-Pro-R...6&browse=-47267&shopBy=-13183&catalogId=-3708
4" version should move the 136" to a 144"
8" version should move the 136" to a 151"
These are designed around moving the 121's to a S/B length but should work on the S/B's too.
Call Tracks USA first to see if they've done it, I'd guess (but don't know) you should be able to get a 1.75" under there no problem.
If you're looking for a 2" you should really just be buying an RMK.
As I found getting the 8" kit and putting it on my 121 indy is not the same as a 144 assault skid and it isn't as light on the front end as you are moving the pivot point (fulcrum) back and the handling charictaristics have significantly different than was designed by the factory.
For your added floatation, expect to get some drawbacks to handling. Everything is a trade-off.