what do i need to do to convert the rear of my 1200 coupe to a 4 link strut setup?

You'll need two coilover shock units, 4 tramp rods with suitable ends, one panhard or watts linkage and somewhere to bolt it all to...

is there some sort of kit i can buy, or can i grab parts from another model - 120y, 180b, etc...

most of this is stuff you'll have to fabricate, you'll need to weld mounts on the diff, and make your own mounting points on the body. It'd be a good idea to take it to an engineer to get the welding done and the geometry sorted out properly so you can get it certified.

The arms could be sourced from something else, Toyota sprinter or Series 1,2,3 RX7 would be good.

I always hang my suspension off the cage, it's the strongest I can make it, and it's only slightly illegal in the 'non-spaceframe' categories...

ok im setting this car up for drag only... i doubt it will ever be road registered.

a CA is going in... can you recommend a good setup for the box, diff, driveshafts, etc? how should the 4 link actually be set up?

You could save a lot of headaches if you bought the datsun 1000 drag car that was advertised recently which already included a 8.15 mustand shortened diff and was setup for a big block chev which will take a ca18, fj20 (a-la-jeff watson victor bray killer sprite), turbo rotor or sr20 turbo or whatever you want and has all the roll cage and the rest.
all you need is engine and box and the car is under a grand with shaiteloads of time and money thrown at it only to not be able to afford a special race 2 speed powerglide. Apparantely the box going to cost about another 4-5 grand!!!. Engineering from scratch is for those in the trade, rich people of drug dealing panel beaters.

Jeff Watson runs in the 8s if I remember correctly. Which isn't really gonna trouble Victor Bray's low 6 sec passes. Bray holds world records for doorslammer racing.