Hi,
that depends on what you want to take from the classic into the 5th Gen. If its only the hard drive, that will fit as it is the same size.
The whole rest is a different story. To make a long story short, most of the parts look like they are the same but they're not the same. If you want to build a 6/5th Gen. mix, take a 6th Gen iPod use the front and click wheel with center button from the 5th Gen. Parts like battery, bumpers,back , headphone jack are the same and can be used in both.