By the time you take all the venting into account, your sound deadening will be minimal at best. When all the power is out and it's silent outside (no city noises, no radios or televisions) your neighbors will still hear your generator loud and clear despite your efforts.
The neighbors should understand that in a power outage you're going to run a generator to power your home. They can just as easily run their own generator or close their windows.
We have a whole house generator, and when it runs we can't hear it inside unless a window or door is open. It sits behind the attached garage, about 60 feet from our bedroom windows. If closing our windows keeps the sound out from a large generator at that close distance, your neighbors should be fine with your generator at even further distances.