I really wouldn't recomend this for any tank that you plan on having hold water because you need to make sure all edges are at right angles and you do a quality silicone job, especially with a tank this size.
I have replaced side panels on several aquariums and terrariums. We purchase a couple of replacement frames (might be difficult for a 70 gallon as it is not a common size and if it is an old tank, they may not make the ones for the thicker brace less tanks anymore). We then pull off the old top frame (which always involves us breaking the old plastic frame in the process). A box cutter works well for sliding between the two plates of glass and cut through the silicone. Then replace the panel, silicone in place (use an angle to make sure the glass plates are at 90 degrees)and let cure. Silicone new frame in place and let it cure.
Again I've only done single side panels before, and most of the times it wasn't even that cost effective, glass isn't cheap, at least not here if you are buying a single small panel at a time.
In your case you are talking about having to cut every piece of glass out and redoing both frames. I probably would just as soon drop it off at a place that builds aquariums and let them handle it if you plan on using it for an aquarium.
