Nice 79! You're a lucky guy. I have one just like it, except not in nearly as good condition. I can't say what it's worth, but with only 40K miles and all original, it should be pretty valuable. I'd have to look at others for sale that are in similar condition and see what the sellers are asking.
It looks like yours might have the WS6 handling option too, since it has 8" wide Snowflakes. Does it have rear disc brakes?