I think that the 5x7 and 6x8 are essentially the same size. Could be wrong, but I do know that 6x9's are much bigger. I would be more concerned about the depth of the speaker and wether or not it will allow the window to operate properly. Before you cut anything, try measuring for depth.
1998 Ford F-150 XLT Flareside, 4.2L V-6, 2WD, automatic xmsn.