The best single thing for a 260Z would be replace your carburetors with early 1970-72 (4) hole dome top carburetors and manifold. I see some are available on the internet if you search hard and long, and they are not hard to rebuild. Also, I experience a 'sputtering' problem with my '71 at higher RPM's (back in 1975) and installed a MSD system that solved the problem.