I am a lot stronger than you realize this is kind of like when the board insisted I was 180lbs when I was 230lbs at the time. I can absolutely squat 405lbs
If you haven`t squatted in years,there`s no way you can now do 405 regardles of whether you still trained legs during that time period......especially with your injuries....trust me,I know what I`m talking about.
315 will bury you also,but you might eke out a sloppy rep or two,but it`s highly doubtful.