I should think you could go ahead and plant them in the ground. You want the ground to be thoroughly warm, and the weather to be hot, though. I usually work up a square foot or two of soil (and do several down the length of a border for repetition), adding in some composted manure and just stirring it around until it's loose and fine. Then sow several seeds into the area, spacing them around. Keep them watered, and after they germinate and form a couple sets of true leaves, thin back to just one plant. (They get huge, you know.) Since your seeds are old and iffy, plant plenty of seeds to hedge your bet.