Answer: Pumpkins germinate and grow very fast indoors and starting them too early puts them at risk for stress -- both for the intense sun they crave and for space. Pumpkin seeds and transplants are generally planted out about two weeks after the last frost date so the soil has had time to warm sufficiently. The seeds are only started indoors about a week or ten days at most before the last frost date. Your best bet might be to work on warming the soil outdoors by using windbreaks, raised beds, black plastic and so on so that they really hit the ground running when you plant them out.
You may find some helpful hints in this FAQ http://www.jkstacydesigns.com/FAQ.html on growing gourds and this one http://www.backyardgardener.com/pumpkin.html on giant pumpkins. Good luck with your project!
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