A real common length back in the mid 2000's was to put a 151" under that sled ('97-02 700 RMK)...all it took was a 151" track, rail extensions, a tunnel extension, longer coolant hoses (only for '00-02 sleds), and a some rivets and/or small bolt/nut combos to attach the tunnel extension. You could even reuse your rear bumper. It was a 4 hour job (some guys could do it in a couple hours and some would take 8) and the cost was very reasonable. In my opinion, this is the best balance of track length with simplicity of installation. Plus the horsepower of the 700 motor makes the stock chaincase gearing just fine.
I did this to a '97 700 RMK (which I have since given to buddy) and a '98 700 RMK that I still have around with exactly that combination (except the hoods are the wedge style as opposed to the gen2 of your '99).
I also have a '00 800 RMK (very similar to your '99) that I run stock Polaris 151" rails (from a '01-02 RMK) and set back the rear suspension roughly 4" to run a 159" track. The work I did to the '00 is quite a bit more involved than had I just put the 151" under it but I like it so much it is still my backup sled.
A used 151"x2" track is reasonably priced as are the rail and track extensions. If you are a savvy buyer you should be able pick up the used package for under $500.
Pictures would be great I was thinking of going 155 but I don't know anything I replavce my skid and track on my 95 formula 585 ( I had help ) so for my rmk it's a 97 would you look for a skid or just buy the extension I like what you did by using the skid off a 900