6 inches with 60 series 185s are generally the best fit, you can fit 7 inch wheels, but they are a tight fit, and require a fair bit of guard massage. Getting the offset correct for 6 inch can be a bit difficult.

I recommend the 14x5.5 Pintara rims. They are a good starting point for a 1200. 185/60/14s fit nicely.

But hardly anyone puts mags on a 1200, that hasn't already installed different struts, lower arms, or some form of front suspension change, so this may explain why the answer to your question is not clear from reading the previous entries.

Working on Datsuns is always an adventure, you have to go find the fun. You might need to do some measuring, and tell us.

Chris