long distance touring: is high tensile steel silly?
Any steel, even hi-ten is less silly than alu for the trip you're planning and the age of your bike. The endurance strength of steel is much higher, and the material itself is not as bad as it's said to be. Also steel is easily and cheaply repairable, which is a serious advantage when you're going on a long tour with potentially few bike shops. But the problem with hi-ten frames is that these frames mostly exist in rreally cheap bikes, where the quality of manufacturing and quality control are quite poor. With a frame this old it means it could crack any time.

Comparing your frame to a new one, it would be much safer to buy a new one. Simply because it's new and the material and joints aren't fatigued (and the fatigue would be much quicker with you and racks full of luggage). But comparing a new cr-mo to a new hi-ten frame, there's not too much of a difference in terms of what they offer for touring bikes. Hi-ten steel has lower tensile strength, but higher ductility, so it's more prone to damage at extreme loads, but will bend rather than crack.
Well in the end it's all a matter of compromise and in case of this specific tour, it would be largely opinion-based, thus out of scope of this site.