Hey,
I have a 95 Searay 180 and I just replaced my 3 step with a 4 step.
It can be done but whata PITA it is:
* remove the rear seats, dog house and anything else
* figure out your wrench size (mine was 9/16)
* fine some metal electrical conduit (the kind that doesn't bend) about 3 to 4 feet long
* hammer flat one end (but not too flat)
* shove or force the open end of the wrench inside and hammer tight (not you have a 4 foot extension)
* put some duct tape on one side of the box end to (catch the nut)
* have a helper stand fast with a screw driver on the ladder bolts
* figure out how to get the box in of the wrench on the nut under swim platform and TURN! (lefty loosey)
* Repeat for remaining fasteners
NOTE: the aluminum metal backing plate will either stay affixed to the swim platform or fall into the bilge...just pull it out when it does.
NOW!! your new ladder holes will probably not mate up to the old ladder holes, so you'll need to patch and drill new ones. I was able to use two of my old ones.
Putting it all back together is much harder LOL!! (no i'm not kidding)
* after the holes are patched and drilled (including the new backing plate holes)
* insert all four bolts and their washers (from the top)
* I super glued my washers to the bottom of the backing plate

* insert the nut into the open end wrench (tape on opposite side helps to keep it from falling through)
* some how figure out a way to hold the backing plate into place while sliding on the 1st nut ( I made new plates which where 5 inches longer than the old ones so I could hold them into place)
* get all 4 nuts on but DO NOT TIGHTEN
* once all 4 nuts are on..lift the ladder up from the platform and 3M 5200 as much as possible even the bolt threads
* slide bolts back into the holes a few times and 3M some more and tighten up
* clean up the 3M 5200
* let it dry and enjoy your new ladder
NOTE: do not attempt in hot summer heat because the sweat dripping in your eyes will hinder your vision LOL!!
Cheers