Once again I see fear from those who have yet to produce an all grain product. I have seen it often on HBT and I for one, don't see a need to worry.
First you have already brewed plenty of extract batches. You know the process. There are a few more little things involved with all grain brewing but in reality, it is extract brewing.
For what you know. You know how a mash should work. If you have your specialty grains for an extract brew, basically mashed before the boil, you have already done all grain. All you did beyond that is add LME or DME. So what is the difference... The base malt. When you create a recipe what is the LME or DME. Essentially it is just the base malt.
Of course you see all kinds of posts related to all grain brewing, and people out there trying to perfect their art. In reality there is no dissimilarity between all grain and extract brewing. Just make it simple. You have already done brew in a bag (so to speak) by doing extract batches. So why not do a brew in a bag all grain batch.
What is different about what you want to brew? The pumpkin. I can easily see how this could complicate your process. In reality there is nothing complex about it. How do you prepare the pumpkin. Bake for 45 min at 375 or so, cut up into little pieces and add to the bag. It can result in a stuck sparge so how do we fix that. Just add some rice hulls. For every problem there is a very simple fix.
To answer your question directly. I would take an extract recipe that you know and produce an equivalent in all grain. Your specialty malts should not change, and all you are doing is adding the equivalent bast malt to match the LME/DME addition. Thats really it.
It really is not rocket science. Its just a baking substitution. If you pick a recipe that you want to do (including your pumpkin stout) just ask HBT what the base malt equivalent would be for a weight of LME/DME. Aside from that you already know how to brew beer.
So jump in head first and just do it. You will learn a lot more along the way as we all have. You will have more questions as we all have, and I would hope that those question will be answered.