Okay so I was just researching the Shadow box for a 75 I bought used and came here first of course.
I wanted to add to the "weight issue" with your pipes pulling on the overflow box.
To combat this, make sure you cut the pipes a bit longer. This basically, for lack of a better term, allows "slack" on the pipes. There is the weight of the pipes/fittings yes; but if you cut them longer, that extra length counteracts the "sinking" or "shortening" of the pipes as they are pulled down because they are taller they provide more support for the box to rest vs being pulled on.
It's a delicate process and definitely should be done with a dry fit first. You don't want it too tall that it's pushing UP on the overflow either. You'll of course get the same effect.
Gaskets seem to also be a good idea so I may do this regardless of which option I choose. Plus, I have a new 150 display and she needs all new gaskets. 8 to be exact. lol
I hope this makes sense.
OH! Wanted to add that if possible, use pipe wall clips or anchors to provide support. These will most likely keep you from having to cut longer as they will keep the system as a whole, from gravity.