Not completely sure what your goal is, even after reading, but I'll offer a few suggestions.
- Check the "plates" thread as Chris at UnderGround MotorSports offers adapter plates for bigger round front speakers.
- Set a budget and let us know so we can give suggested packages that fit within it.
- Check your local Car Audio store and listen to some 6" coax's.
- Let us know what they were and what amp the store was using to power them.
- Decide exactly what you want - 4 full range speakers? 2 full range & 1 sub, 4 full range & 1 sub?
Off the cuff, I'd say a good mid to upper range Pioneer coax's in the doors, a simple 10" sub and a mild 4 channel amp (stereo front, bridged rear).
Pioneer TS-C1653 - $119.00 6 1/2's with tweeters and crossovers included.
or
Kicker K65.2 - $129.00 6 3/4's with tweeters and crossovers included.
With
Phoenix Gold Titanium 10" Subwoofer - $89.00 more sub than you'd need, but on a deal.
or
Kicker Comp 10" Subwoofer - $49.00 Simple basic sub.
Using Phoenix Gold Octane-R 5.0:4 - $159.00 Simple 4 channel amp
or
A nice small Autotek amp, I'd reco the SX line, they have 4 channels available on ebay for about $200.
This is just a down and dirty reco, let us know more of what you want, and I'll try again

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Good lcuk.