From my experience turning 10 and 20Long aquariums verticle I would say it is going to be very difficult. Possible- Probably. A couple of good questions-
1. Are you going to make a glass front?
2. Are you taking the glass out of one end and putting in screen at the new "top"?
This is the first tank I tried. The picture was taken before I glued the screen on the new "top" end. You can see the glass front with latch and hinges. A couple of good tips for you. I used 1/8" glass for the 10's and 20's. It cracks very easy. I tried make a front for my chest/dresser turned gecko enclosure out of the same glass and shattered it while I was trying to glue it in place. If your doing a similar job, go with thicker glass. (Or acrylic)
Secondly, I have had some issues with the silicone holding the glass pieces in place. It seems that over time they start to come loose from the plastic rim of the tank. I have somewhat solved this by glueing them to the tank, then the next day running a bead of silicone on the outside to provide some extra support. With a 75 gallon I would try to figure out a way to put the glass pieces inside the rim of the tank. Also, my top and bottom section of the front(where the hinges and latch attach) are 2" tall. With 2" of water in the tank I had the glass completely come off. I need to get with some dart frog people to find out what they do to stop this from happening.
If you want to try another forum that could be helpful you can try frogforum.net or dendroboard.com. I just figure since the people doing darts usually have to deal with a lot of weight from the substrate and water in the tanks they might have some good advice.
Another thing you can think about trying is getting some window screen frames and building a screen front. You could probably screw some hinges on as well(to the screen frame) and silicone the frame to the tank. Since this will be so much lighter in weight you shouldn't have the issues of it coming undone.
The most important thing has got to be your end intention. Without knowing exactly how you want this enclosure to function it will be hard to give good thoughts on it
