I resealed my 160g waterbox, and the sump.
It is not hard, well the sump was hard, the tank is actually very very easy.
You want to make sure you use the right silicone. I really recommend the Aquascape Pro. It is a few dollars more, but when you have worked with it, and then something else, you will notice a huge difference.
It is just like painting. The prep work takes a long time, the silicone part is quick.
Melev has a cool video on youtube where he documents a tank maker making 60g cubes. I watched that and followed it. No clamps or nothing just some masking tape.