If you're reinstalling your old injectors, you should replace the one that was squirting into the cylinder that failed. As Brapple mentioned, it looks like that cylinder went lean & melted the piston. The is that you don't know if there was a problem BEFORE the piston melted,, (like excessive blowby from high boost), which pressurized the crankcase & popped off your PCV tube, which caused the lean condition that melted your pison... OR, if there was an initial lean condition, (say from a bad injector), that led to the melted piston, which allowed enough blowby to pressurize the crankcase & blow off your PCV nipple, which would then, exacerbate your lean issue. IMO, the latter is more likely... but either way, I'd replace that injector.