The aftermarket radios i've seen that fit in the original holes always seemed to be on par with the $19 Kraco or Spark-o-Matic radios you can buy at Walmart.
Some folks will retrofit an original radio with AM/FM/CD/iPod features, but i've not ever heard anything stellar about their quality or sound, and those retrofits can cost a bunch.
Spanky linked in the General Truck section a link to an outfit that makes what is supposed to be an original looking (sans Chevrolet script) radio but from the pictures it has a VERY repop (read CRAP) look to it, and has a $500 price tag.
I say put the "radio delete" panel in your radio hole, and put a balls out Pioneer stereo in the glove box and call it a day.
A $250 Pioneer head unit will have better sound and features than a $500 sorta retro radio.
But that's just my opinion.
