There's a good BYO article I have used here
regarding fruit beers with some really useful info. I've not worked with elderberries in a brew before, but I imagine the process is just the same as with other fruit.
You may want to reduce the corn sugar (to 4oz?) and increase your extracts (1lb each, maybe?) to get some additional body in the beer. Also, I'm not sure if I missed it, but you may also want to add some bittering hops in your boil unless you want it cloyingly sweet.