The rears are indeed 5x7 and 6x8s will not drop right in. Honestly, I would not worry about the rears at all. The front speakers are 6.5". I would suggest taking your budget for the rears and just getting better fronts with an amplifier. I would shop used with your budget.
My stock fronts do not look like 6.5". Are you sure that they are that size? What does it take to drop in 6x8s?
1990 Mustang GT
306, typhoon intake, E303, stock heads, 1.6 FMS R.R., 70 T.B., 80mm pmas, 42lb injectors, 255 in-tank fuel pump, adj. billet regulator, MSD dist., coil & 6A box, BBK shorties, o/r x-pipe, spintech mufflers dumped, T-5, 3.55's, e-fan, b&m pro ripper, S-Trim, 396.37hp/398.06trq.