I had the OEM radio in my 72 Grand Prix converted. Full repairs and conversions run minimum $400-$500. Hardest part was removing it, lol. As far as speakers go I have 6x9 Kenwood 5 ways that fit right in the original spot. Here is a link with several repair people that do the conversions
https://www.tech-retro.com/aurora-de...o-dealers.html