For what it's worth, I measured the exposed seat post on my two modern road bikes and found 4 inches on the Veloce and a bit less than 5 inches on the Pista. The Pista has a high bottom bracket since it's a track bike so it's a 2cm shorter frame size than the Veloce to get the same standover hight so the seat is higher. My vintage bikes expose about 3 1/2 inches of seat post. All of this is measuring just the post portion not the clamp so add about another inch for the clamp if you want. I would expect a 1990 bike should be set up something like my modern bikes with 4-5 inches of seat post showing. Functionally 6 inches is probably fine if it's not above the max extension line. As to whether it looks dorky, only you can decide but I suspect it's probably fine.