Here's the deal. A 5.25" speaker will fit perfectly with an adapter plate. Crutchfield has them for $10, or free with 5.25" speaker purchase. You can fit a 6.5" speaker in your stock 5"x7"/6"x8" speaker location but you are gonna have to make your own speaker plate. I've looked around and I can't find anyone who makes a 6.5" plate. What you do is get a piece of MDF and cut it to the size of the stock opening, but it needs to be a little larger, here's why. The stock opening is 6"x8". The 6.5" speaker is 1/2" larger so it would apear as if it wouldn't fit. The plate you make must me oversized so that you can have room for the larger width. Basically you'll cut a 7.5"x"8" rectangular piece of MDF, (make sure it's about 3/4" of an inch thick) After you've cut the speaker hole for the 6.5" driver, screw the new speaker into the hole you cut. The extra width of the MDF should give the speaker clearance in the stock location although this could change with different speakers. Then, once the speaker in the plate screw the plate into the stock location using the 4 screw locations for the stock speaker.
Sorry that was kinda confusing but I hope you understand. An oversized 6.5" speaker maybe pushing it a little. The size difference in the speaker isn't in cone size but in the size of the basket. Some cars, such as Chrysler have these oversized openings and the extra basket diameter is needed.
Hope I helped and sorry so long.
Steve
