I'm with Savorykitchen on the bechamel. I don't know why any pasta al forno would call for heavy cream in the first place unless the author of the recipe is a total wimpola taking short cuts. The Italian term is balsamela, and essentially it works as a binder with baked pasta. I'm guessing that the cooked pumpkin gets beaten into the sauce...

Creme fraiche works

Canned evaporated milk is a good substitute because it has fewer calories and fat than cream or half and half, but it has the same "feel" as cream, and it also has a nice dairy taste.

I grew up in several countries where cows were scarce, most often used for meat instead of dairy. Evaporated canned milk and powdered whole milk were pantry staples back then, used as coffee creamer, to make mashed potatoes and, yes, even whipped cream. I never liked powdered milk, but I keep a can or two of evaporated milk on hand in case of an emergency. In fact, I prefer canned milk over cream in potato soup.

I think you can use half and half, as Merrill mentioned in the recipe comment thread. To keep the creaminess without cream, you could also try making 2 cups worth of a bechamel with milk (or even vegetable broth) and subbing that in for the cream.

Half and half should work!