Dovetails are nice, but get them wet and they will eventually fall apart as the wood drys and shrinks. Finger joints would probably stand up to moisture better than a dovetail would, but they can be a pain to cut. I've never seen a tank with a wooden fram like what you are proposing, so I don't know how it would act over time with the occasional splash of water and it subsequent drying. It COULD eventually warp and break the glass, if you use glass, or bust the seals on glass or acrylic. But like I said, I've no experience with building aquarium frames from wood. As for using the front and rear walls of your 55, the glass would definitely be way too thin. 1/2" glass would probably be good, tempered 1/2" would definitely work.
