spindle diameter is the same, the difference is in the spline machined in the end. viper/apex/rx1 all use a finer spline, all other pro actions, use the thick spline. Any pro action non viper that uses long tube trailing arms, uses the large spline with long spindles.
I have both versions
you can also use apex spindles in srx short spindle trailing arms. viper and rx1 spindles will also interchange minus the snap ring.
the cranks that go on the fine spline are different between the viper and the 4 strokes, though they are the same between the apex and rx1
drop brackets decrease the handling of your sled on the trail, and in the powder will make it hook better. Your only going to a 144 so my guess is your not off trail that much. I wouldn't bother if your going to ride on trail mostly.
if you are going to drop the back, you definately will want to raise the front as the sled is going to rake down and really look like you just blew your shocks