I've seen tachs for sale that have the red or green light attached. Is this to tell you when the best time to shift to next gear is? Also, The last time I drove a stick was over 30 years ago, my dad's '74 Ford Pinto. I'll have a freshly built 383 stroker w/TKO 600 in my '68. I guess that'll be a little different???

Yes they are shift lights that go on at your selected shift RPM.

mark, which tach are you looking at and where are you going to mount it on your 68??