A guy in our club had a 450 tank. He used a mag float on that. I think he had .75" glass, though I'm not positive. I do know that whatever float he had on that thing you couldn't MOVE it on his tank!! It was a struggle to move around. There was definitely enough pressure on it and I can't imagine what would happen if you put the two mags together on the outside of the tank. I don't think you'd ever get them apart. I'm thinking you'll find something suitable as long as you check the specs before purchase.
Good luck! And hey... how are all of those fishies?!