What is your trim level? I think that only certain Legends come with stock 6x9's in the back. They should accept 6.5's easily but if you must have the 6x9's find somebody that is parting out a LS Legend with the 6x9's in the back and ask them for the plastic containers that hold the rear speakers in. This is a plastic basket for the back deck that adapts the 6x9 speakers to fit in a 6.5 hole. Either that or make custom boxes in the back for them.
Yup, Bose system Legends had the 6x9 in the back. What do you mean that they don't fit? Are they too deep? I had this problem when I tried to install Alpine 6x9s. The stereo shop decided that they would give me something that fit for no additional charge. What happened to fit was a set of Boston Acoustics 6x9s that were more expensive.