I totally agree with @IslandLizard . In the mash is the easiest way to go. I use ≈ 3.6lbs/5 gallons. I think that’s 4 of the 15oz cans.
Pumpkin beers are more about the spice additions than the pumpkin - coriander, cinnamon, clove, nutmeg, and ginger are the usual suspects. You can add them...