It all comes down to this: do you like the way the Trek rides, and does it fit you comfortably?

If it does, then there's your answer... keep it, and build it up the way you want it.

Personally, I'd take the 620 over an LHT any day of the week. I have an '83 620 also, built up with an 8 speed Shimano drive train.
Old school Deore XT, friction shifting and it rides like a dream. Of course, on your frame the crown jewel is that Sugino AT crank set.