Hey Steve,
I have a 98 ford f-150 supercab,but it has 4-7x 10`s...not 6x8`s,they were originally stocked with that size aswell...
I had purchased the XLT-STX model,so im unsure if that is why they are different sizes?
Either way, i replaced all 4 of the Speakers with Clarion 3-way Speakers and added a 400w clarion Subwoofer and Amp.
The CD player is 4X50 output so it is slightly underpowering the 4 speakers,but sounds crystal clear.
Beyond this the addition of a Lightning Audio 400w amp (The Bolt) is a excellent Quality self cooling amp,and Clarion 400W Dual Tone Sub made for an overall nice sound with some nasty kick to it.
HTH with some more ideas.