Sorry I didn't reply to this sooner but I saw you were looking for a pumpkin porter recipe and I don't have one but I do have experience with making a pumpkin beer.
Here's the one I made and I think it is a decent beer.
http://www.homebrewtalk.com/f76/richards-pumpkin-ale-23489/
From the research I did I determined that the pumpkin needs to go into the boil. There aren't enough starches in pumpkin to warrant mashing. I used 2 large cans of pumpkin straight from the can and didn't bake it in the oven before hand. My batch was actually 6 1/2 gallons because I let as much pumpkin as possible get into the primary fermenter. When I racked it to secondary I ended up with just over 5 gallons. I think the pumpkin flavor is just about right.
Use your recipe and add the pumpkin to the boil. Being a porter, you may want to increase the spices somewhat because they might be muted by the roasty/malty flavors.
Good luck and let us know what you decide to do with this.
