S30 owners, generally, prefer the early cars to the 75-78 models. Early cars have the sexy bumpers, they weigh less, the horsepower is more or less a wash, and the interiors are more raw and purposeful.
But the later Z cars benefited from a LOT of on-the-fly improvements, namely, to chassis rigidity and stiffness. Sure, that came with extra weight, but a stiff chassis is worth a slight weight penalty.
Ideally, if one wanted to build a resto-mod Z, you'd be hard-pressed to find a better platform than, say, a 1975 model. Significantly more chassis stiffening than the earlier cars, provisions for fuel injection, but without the bloat of a 77-78 model.
Be advised, I'm already anticipating hate mail from everyone with a 1970-74 and 1976-78 Datsun Z. Don't bother - it's my story, I'll tell it how I want.
With that in mind, while in San Diego for ZCON 2014, I purchased a pristine, low-mile, 2-owner, rust-free 1975 Z from a long-time Z enthusiast and collector in Southern California. At the time, I felt I overpaid somewhat. In hindsight, it was a steal. For five years, it would sit in the garage at Datsun Ranch, awaiting its eventual fate.