cd player box

plenty of companies make trunk (remote) mounted cd changers, but I fear that none will interface with a 73 datsun Head unit

WAIT!

You have a 73 datsun head unit that still works? are the original speakers still intact? maybe you should talk to the curator at smithsonian

seriously, though.. that's a tough decision between preserving the looks of the original, and kickin some sweet tunes..
IMO, you'd be better off getting something newer if you want cd capabilities. But, you're right.. tough to find a good stereo install kit, I've used generic kits, and "engineered" them to work, but that depends on how good you are at fabricating. aftermarket under dash kits are available, but tough to make look stock, and that's a little car, so naturally room is an issue. I have heard of companies that will take the vintage radios and gut 'em, and put new electronics in them, but where to start looking is an interesting question.

I think that you might be able to run a changer in the trunk, and maybe then run an fm modulator from there, but you'll have to get the attention of someone wiser than me if you want advice on how to hook that up, (I've never the endeavor).

 

You might also consider mounting a unit in the glovebox, It would be more easily accessible than one in the trunk and you could hide it quite easily. Post some pics of your car on here, I love old 280s and 260s. Although I drive a 300, I still love the older z's.

 

you could use an fm modulated cd changer if your factory radio works and is fm capable.

otherwise you could get an undermount kit like this one

http://www.bizrate.com/marketplace/search/search__cat_id--11950000,prod_id--347748572.html

you could mount the radio in the trunk and get a wired remote for it so you can control it up front.