Thickness is related to the height of the tank, not the total volume (length isn't really important when braces are used). Seeing as how the thickness of the 75g (20" tall) is the same as your 55g (21" tall), I don't see why it would be a problem.
I'm assuming you meant to say 3/8" which would make it 50% thicker than the OP's tank (if it was 3/16", that would make it thinner than the OP's tank making your statement about the old tank having thicker glass incorrect). The older, non-tempered tanks were thicker and I believe did not include a brace. New tanks are built thinner because they use tempered glass which is stronger. Thicker glass is still used on rimless tanks though to compensate for not having any brace.