You need to check locally and see what engines are selling for. 1 word of advice, if you sell it leave it in the truck for now running and sell it so people can see and hear it run, that will always bring you more money if they can run it and test compression and so forth.
In my opinion, if I was looking at it and could actually see, hear, and test what I wanted, for a stock 350 that has been untouched, I would pay $500.00 - $700.00 depending on condition, if I see any evidence of previous leaks and so forth that would determine where in the price range I would pay.
Like I said, if at all possible, sell it now if you want while in truck and still running, will bring more money.