Not entirely sure here, because I don't have exactly that setup installed, but you should have at least 5 preferences files starting with com.stuffit.(something-and then ending with).plist.
- That's in Sytem Folder/Preferences
You will have for sure "StuffitEngineShell" (in System Folder/Extensions), which is part of 7.0.3, as opposed to the old Stuffit Engineā¢ extension.
There's probably a True Finder Intergration in System Folder/Control Panels. (It might have been renamed otherwise after 6.5, but look for something aladdinely stuffitee looking icon.)
There's also an item of interest in System Folder/Contextual Menu Items.
But your real concern is probably located in Sytem Folder/Application Support, where you would find a folder (in my case it's Aladdin), where you will find a Stuffit and a Stuffit Express folder. The "stuffit" folder is where all the Magic menu items and extensions used to be.
Just do a command+i on those system folder items and the information window will tell you if the item you are looking at is related to 7.0.3 or 5.5. (That being said DON'T trash the extension folder called Stuffit Plug-ins, even if all its items are said to be 7.0.3 items.) I mean this because I can't assure you that the 7.0.3 installer didn't replace old 5.5 files which would just make sense.
Obviously the simplest thing to do is just to command+f in finder, which launches Sherlock and search your system folder with it for "Stuffit" then for "Aladdin", and do a command+i on the items it finds. (But like I said don't trash a file that is located in the Extensions/Stuffit Plug-Ins folder... unless your plan is to reinstall the app)
Or you could do such a search and trash everything you find and then reinstall 5.5.
I hope this was helpful.