The joints will very strong if all of edges are water tight. When the tank is sealed I add a piece of plastic that is one inch wide around the top. This piece of plastic makes the sides stronger so they do not bow when the tank is filled. You can see this top molding in the pictures of the finished tank. There are several ways to finish the edges of the tank.
Sand paper is the safest way to round the outside edges.
I use 200 grit and then finish with a 400 grit wet and dry paper. You can also finish the edges with a blow torch, but I am sure you will want to practice on some scraps first. Let the tank dry in a warm room for 24 hours before you fill it with fresh water. If there are any leaks you can apply a bead of thick cement to the inside seam. There are many ways to plumb your tank.