If you are looking at using an original color, there are plenty of sites that have original paint-chip pages to look at. You can import them into PSP (been using it for 10 years now) and modify slightly to your desire.

Also you can scan an original piece (paint-chip, panel, accesory, etc.) and import it into PSP and play with that as well.

Getting a NAPA guy or other paint stud to mix a batch of original paint that matches an original paint-job is pretty darn hard, impossible actually. So all shades will come out slightly glossier, slightly deeper than current 36-38 year old paint jobs. If you are truly re-painting the entire car, these minor differences won't matter.

I'm stuck in this particular dilemma right now, as my 68 original April Gold paint looks like it was painted last year, but a few important spots need new paint. Can't match it perfectly, but want to keep original paint color. It sucks to have to probably eventually re-paint the whole car just to fix a few spots.