Deck and Sub Install on an 82

[metal chevy]

Just bought a new to me 82 k20. Right now it has no deck or front speakers, though it has the 2 speakers up in the dash. At this time im not sure if they work. The dash where the old stereo was located has been cut it looks like. Ill upload a pic of the cutout. I can tell im going to have to rewire most of all of it because the only the wires i can find right now are the dash speakers, as well as a grey wire and black wire. But if the dash is cut to fit a newer single din deck, will i just have to get the wiring harness and piece that slides in to house the deck? Would i be able to find both at an audio store for this old of a vehicle or will i have to order offline?

Also, does anyone know where to buy a 12" sub box to put behind my seat? Ive looked around online and most sites are pretty vague on fitment. The 12" i have is an Alpine Type R if that matters. Thanks guys.