my previous samsung 50" plasma was 400x400
my new sony 65" led is 300x300
my bracket fitted both, but when mounting the sony, i had to adjust where i mounted it, as the fittings were at the bottom of the tv, rather than in the middle.
brackets can be bought very cheapily.
mine was 15 quid (ish) around 4 years ago.
there are plenty on amazon