i've had a clarity plus acrylic 125 in continuous use for 13 years, and it was "well used" when i bought it. it bows about 1" in the front, and has since day 1. never had a leak or crack yet (knock wood). my back is a single clear sheet, not stacked thinner sheets.
at one time there was a huge buttikofferi in that tank who would spaz out and bash the ends relentlesly whenever he saw our ferrets, no problem.
i like the previous ideas of reinforcing using wood or metal to frame out the tank.
also, the idea about cutting the top 6" or so and then using the existing top bracing could work nicely. check with a local acrylic shop to see if they will cut the sides and weld the top back on for you.
maybe try filling it to the very top outside and leaving it setup for a few weeks to see how it holds up. that should definitively answer the question of whether or not it will hold up!
best of luck.