If it's the 181 series or higher, it will be adaptable to the aftermarket scope mounts whereas the 180 series is not. This may add a few bucks to its value although I've never heard of an aftermarket mount that would hold zero.
The other guys with them might be able to offer advice if they have a mount that works.
You might want to check with Ruger as to what twist the barrel has. I want to say that Ruger has used 2 or 3 twists over the years, but I'm not sure about that. Don't know if would increase value or not, but would be info that might help you with a sale.
Pricing the closest model locally like at WalMart will give you an idea as to price for a new one. Any weekly shopper classifieds papers might give you an idea on the current market.
1979 should be preban so you can hang a folding stock, flash hider, pistol grip, and bad assed bayonet with a pair of red panties hanging off it. This should add a few bucks to the value.
Going to a show tomorrow and will try to remember to look.