It seems like it's deep in the bowels of your boat, but once you remove your cleanout tray, access is simple, and you should see that it will be no problem for you. As long as you have a wrench to fit the new scupper, and can tighten everything down nicely, you'll be perfectly fine. Get yourself the wrench, secure a helper to make things easier, and have at it. Here's everything I used to swap mine out:
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Mark the top of the scupper on the threads/inside part of the scupper so you know the proper orientation when looking at it from the inside. See my blue mark:
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On the outside ring of the new scupper, mark whatever you need for orientation if you have a helper.
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I used a pry bar to hold the scupper in position. Test the fit of whatever you're going to use before you ever add sealant to the new scupper. Here's a look at how the pry bar was used on my scupper to hold it in its proper orientation while tightening the nut.
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A helper is NOT required to do this, but it WILL make things much easier. Only thing I didn't show is some paper towels to clean up squeeze out, and stuff I used to remove the old sealant. I'm no Kareem Abdul-Jabbar either, if my measurements are correct, I'm a 33, so you can do this!