This should be easy..Contact These folks:
http://www.navionsociety.org/old/index.html , they should have info on Navion STCs.
I fear you might be a decade or two too late, a lot of STCs that were around in the 60s-70s are no longer available, but the club should know what's current. Who knows, the plans may still be out there and you could use them to fabricate the parts you need.
Also, it there is one on your field,you might take a look at an early model Cessna 310. IIRC, they had a very long step that may have been telescopic.
Looking at one might help you in coming up with a design if an STC isn't available. You could get your mod approved with a form 337.