What is your standover clearance on the bike? I think older fit philosophy called for less top tube clearance than is typical now (try finding a 25" or 27" frame made today!). Along with this, seatposts were shorter. Mountain bikes hadn't came along yet to popularize 350-400mm seatposts, most older seatposts are significantly shorter than what is sold today. I really suspect that when the bike was new, you would have been directed to the next larger frame size.
Just pick up a new seatpost. $20 or so should get you a basic post off ebay likely with free shipping.