I know this thread is old but I looked it up because I had the same problum, I undid all the bolts and the platform still would not come off. I didnt really find out why by reading it. I found out it is because on some of the boats, they used longer screws for the rub rail, the screws from the rear rub rail actually went so far threw, they go into the lip of the silent thunder.By removing the rear rub rail screws it drops right down.
I have a 1993 271 SR-1, I put flappers on and tried it without the silent thunder, my carb'd 502 mag blew a 92db up from its lake acceptable 80.6.
Platform is going back on so I dont get harrased, the bad part is now I will be able to hear my wife yelling, slow down, again.

Someone else had it off and apparently used silicone on the inside when they reattached. Ended getting off but took some time. I love it without it on there.

I had mine off of my 336 that has 525 SC's and let me tell you if you want to scream to hear the other person thats what your going to get. At idle you can not hear the person next to you. Plus if you want to put exaust tips on to make it not as loud you have to be careful with those motors you can't just bolt anything on because of back preasure. These motors have to breath and if you stop to much water on 500 & 525's you can hydrallic the motors. If you only plan to run 2000 rpm you would be ok with any tip but trust me this is not an easy fix to make it not so loud. Mine without the box was 126 dec and should be 90. The proper mufflers are abour $1800 for that boat with those motors. I worked with Formula on this and not a good idea, leave the box on. My boat with 525 sc motors with the box was running 96 dec. so I made my own inserts to get it below 90. If you send me your email I will show you what I did. Plus in Illinois if you get stopped for loud exaust you can not put the boat back on any IL water until it gets checked again. Good luck.