Great question. If you want to keep it looking "vintage", there's a company called "retrosound" that does some really neat classic-looking head units with modern "guts." Actually, if you do a Google search for "vintage car radio", you'll see several companies are doing this.
Occasionally, you might also run across a vintage unit on ebay - but the early Datsun radios weren't much to listen to.
If you decide to go modern, the sky is the limit, but be aware you may have to do some dash trimming.
Personally, I'd install a modern head unit in the glove box - if you get one with a USB reader or an aux cable input, you can hook up your iPod, MP3 player, or even play tunes through your phone.
Looking forward to seeing some pics of your 520!