Hard to say. You had that nice fence around them last year
but at night it might not show as well with jacks in front. However, I'd think in front of the fence would be good or a bunch all along the back might be a nice as well. Although at 20ft back the detail in the pumpkins might not be appreciated.
You could also make a pumpkin path that leads to the cemetery. People could walk along and enjoy the jacks when all of a sudden, they find themselves in the middle of a Zombie nightmare
How many pumpkins are you thinking of doing?