6.5's are your best choice or aftermarket 6x8's (OEM size)... 6x9's take a bit of trimming and modifying but can be done if your determined as long as they are not overly deep... not much room to work with behind the mounting points... My 97 and 98's both run 6x8 replacements and sound excellent. my 2000 has 6.5 components with removed tweeters located on the dash area, and 6x9's in individual boxes. sounds great easy to put in... all personal preference tho.
The only way, Is all the way!
1997 F150 4.2L w/Auburn Posi 3.55, True Dual w/X-Pipe and Glasspacks, Navigator Buckets and Console
2000 F150 5.4L 4x4 w/3" Body Lift, Cranked Torsions, 315/75r16 Toyo Open Country MT's, 4.56 Gears, Auburn Posi, Custom Fabricated Bumper, Custom Floor Shifter Conversion
1998 F150 4.2L w/Dynomax Silver Bullet