I found this interesting: The design of the 300zx is more curvaceous compared to the other nissans (180sx, silvia etc), hence the difficulty in getting the two-tone paint to look 'right'. Its a good read, if anything.
my idea is like yours GOTTAGO except just following the window line straight ahead until it joins the bonnet. but i think it could look stupid where it meets the bonnet because there will be an angle there.
That little yellow triangle at the top of the fender is the problem for two-tones. Screws everything up aesthetically and the sharp edge makes the two-tone zed look like it's been made out of spare parts. Quite disappointing for those who want two-tone, but it enhances the looks quite a bit in a single-tone car, with black zeds being the only possible exclusion due to the darkness of the paint making the edges quite difficult to see. Just my 0.02, so feel free to flame if you think I'm talking bollocks
here I go trying to prove you dont need to touch the guards to have a nice blacktop Z I have seen lots of nice example black tops, seems to be hard to find pic's though, just depends on how its finished, most of the pics i seen doing a quick search were complete stock + bad condition with black top. so they are nothing to go by!