Although the High Sierras are finely made mtbs, I think the early 80s models will have extended chainstays and a long wheelbase. Also very relaxed angles. It may make for a very lazy riding tourer.

The later ones will have more sporting geometry and shorter wheelbases. The top tubes though, will become longer.

As for size, if you can straddle a 61cm road bike at 5'10", your inseam must be 34"? I think a 21" mtb frame will fit you nicer. I'm 5'10" with a 30/31" inseam, and the 18" mtb frame is my minimum size for comfort.

BTW, the asking price does seem a bit on the high side.