I've put up a bunch of Zimmatic 307s, and have shortened a number of those spans, which is easy to do.  The standard span length is 177 ft., and leaving one pipe out takes it down to about 135.  A 410 model will have shorter 125 ft. spans to begin with, if that would fit your needs.  I believe the G2 pivots should be simple also.

I've helped someone do a Valley 8000, which was a bit more complicated, had to buy more truss angles to shorten a span.

I've bought a number of pivots from Tom Johnson in Nebraska.  I haven't spoken with him for a while, maybe someone here will know if he's still around.  You could also check out Pivots Plus, or auction like Bigiron.  Just make sure you verify condition, many machines are rusted out junk.