Sounds like you don't know what you really want/what things really were back then.
French fit road bikes generally do not work that well for someone with average dimensions. If you've got short legs and long arms/torso that fit style is ok but if you're more proportional FF is going to either leave you stretched and uncomfortable or leave you stuck with a super short stem that makes climbing sluggish and slower.
If all you want is a modern bike with a horizontal top tube, just buy the Rivendell Roadie.
but this is a question about what's the difference between an 80's mid-upper steel bike and modern ti or steel
Nothing, expect superficial tubing differences or differences specific to model. Look up some geometry charts and you'll see most of the angles/tube lengths are close enough to be the same or are actually the same.