ok we come into a few different issues here and they are upto you if you take the risks involved... a normal circular saw with a straight edge guide would do the trick then comes the reattachment of the end piece ....
i would only attempt it with a product called weld on ... or a really good epoxy such as lock tight (both are aquarium safe)..
now we have to look at the integrity of the wall once repaired... i am making an assumption here that there is a rim of some sort involved and once you cut the tank itself you have a rim that is to big...
one option is to use Euro riming (Google it to see what i mean) another option (less trustworthy in my oppinion) is to cut down the rim and put it back on backwards using the weld on product to secure it that way you have more surface bonded away from the new seal.
the last (and most dangerous option) is to run it rim less which will most likely bow and break the tank of that size.....
another would be to build a wood frame around the top and bottom but there are always some risks involved such as measurements and fasteners..
in short we it me (and its not) i would recommend making more room or getting another tank....
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