Will a 2005 Chevy siveraldo cap fit 2008?

No, a 2005 Chevy Silverado cap will likely not fit a 2008 Chevy Silverado. While they're both Silverados, the bed dimensions changed slightly between model years. The 2008 Silverado likely has a different bed length or a slightly different bed shape, making the 2005 cap incompatible. You need to check the specific bed size (short bed, standard bed, long bed) for both trucks to be certain. Even if the bed length is the same, subtle changes in the bedrail design could prevent a proper fit.