I should be getting my 1999 Mustang Coupe this week!!!!

(Sorry, i couldnt resist yelling that out loud) A couple questions: First for the Mustangs with Mach 460 sound, i know i need an adapter to hook up a new headunit to the amps, but has anyone actually done this? Does it degrade the sound? Having just bought the car, replacing the amps and everything isnt too much od an option at this point. I would like to use the headunit i currently have.
The second question is more general: Has anyone successfully put a 20x4 or 40x4 character LCD in a DIN sized housing? I currently have my LCD in a radio shack case, and i am debating if it can fit in a DIN slot. Thanks so much!
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Car: 1999 Dark Green Mustang GT Coupe
Audio: Alpine HU, JL 6ch Amp, JL Stealthbox and XR series components.
Player: Empeg for now; something custom later.
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