I've tried that with 60ns ZIPs ages ago - with some errors being detected every now and then. I guess if you have 60ns with a bit of error margin your set - unfortunately there are no 50ns ZIPs. A solution might be to use a PS/2 adapter and 50ns EDOs - rare, but they do exist. If you can live with 4 MB fast, get a set of 44400 DIPs in 60 or 50ns and try those.
