Where do I start...I'll try 'the start'...
510 ones won't fit because of the spring diameter. They also won't fit because the balljoint is different between these models.
Get Stanza Lower Arms and control rods. They use the large balljoint that allows you to use 'whopper stopper' 510 brake conversions. Take your 1200 ones with you to the wreckers. This way you'll be happy that the version of stanza ones will fit. Also with Datsuns, you have to be weary that even though parts are on a wrecked car..doesn't mean the factory put them there...
Once you have them, You'll need to cut off the cups from both your new struts and your 1200 struts and weld the 1200 ones onto the 180B/200B ones.
Word of WARNING. The 1200 struts are smaller diameter than the stanza/180B etc.. Best bet is to get some cheap Stanza struts, they have the same diameter spring size as the 1200 ones, but the strut is larger in diameter. This makes the spring cups from the Stanza more suitable for the transplant. You'll need to mig weld the cups back on. Use a hacksaw and angle grinder to remove the cups from both struts.
You could also get 260Z struts, they use the small springs and mount on the stanza lower arms. I know this because I've heard 240/260/280 Zed car struts have been used in 510 conversions. The problem with using the zed struts is that you are restricted to Zed aftermarket brakes.
I recommend you get 180B large axle struts.
Avoid the scalloped hubs as these don't allow for the Commodore/Verada conversion. See:
http://www.shoeys.com/content/datsuns/datsun2/lnew_23.jpgChris