One would have expected Prestige to provide some advice, but without it, I recommend you match six-inch vents with six-inch ducts. Larger ducting reduces resistance but also slows down the air velocity. Commercial ducting requires 2000 ft per min air velocity to limit the amount of grease that will condense. According to some sources I've read, residential velocities should aim for the 1000 to 2000 ft per min range. The two six-inch vents can be merged into a duct with the same cross-section, around 8-inches, but only if both power packs are powered at the same time. Otherwise, keeping the velocity up requires individual ducts. A call to Prestige may be desirable. kas

The assessment of trades above by @User is correct. You’ll need a HVAC person, an electrician, and someone to do drywall. You’ll also need a new dedicated electrical outlet for where you moved your MW. I’m not sure why there isn’t an outlet above the range, where you moved it from, but it wouldn’t be in the right place for a chimney hood if it were. That’s why you have to decide on the hood, and design everything around the exact hood’s specs. The location and shape of the duct and the electrical location is NOT universal. It’s normal to have to move or adjust everything, or do new. You need the specs and the hood on hand when talking to contractors. Don’t forget an exterior damper to keep cold air from infiltrating your home! You will get a better job, done faster, if you hire a General Contractor, rather than trying to hire the trades individually. They have their subs that they give lots of work to, and who will return phone calls and show up. And pull the permits that this will require. A homeowner doesn’t get the same price or response from the trades, and it takes weeks longer. I’d suggest posting on your local NextDoor or Facebook neighborhood or city group to look for a licensed and insured (check this!) contractor who can do a small job. But, it isn’t as small as you think!

I'm not sure I understand the intent of some of the comments. As I stated in the original post we blow hot air out of the upstairs windows while simultaneously opening all the downstairs windows to allow cool air to replace it. This method fills the house with cool air from the bottom up in the early morning when the outside air is coolest. I considered a whole house fan as we installed this in our last house and it worked quite well. Same principle - open all the windows downstairs while the fan blows the hot air out. I'm not sure how this got turned into an issue with not getting enough fresh air into our homes - we live in a mild climate and open windows to bring in fresh air all the time. This house has blown in insulation in the attic that is very light and is easily disturbed so I fear that an unducted whole-house fan would eventually cause the insulation to clog the attic vents. I suppose a ducted whole-house fan would work and I understand the reasoning behind separating functions. Since it seldom gets really hot we typically only do this for a small percentage of the days during the summer and it seemed to only have to run for about an hour so I thought it might be a good case for dual purpose. I just thought there might be some clever solution that could accomplish this without a lot of effort and expense. Thanks for the input so far.

"The manufacturers of Vent Hoods are miseducating the consumers." Yes. Somewhat. Not really. They are in business to make money and they do that by making hoods as inexpensively as possible and then selling as many as they can for as much money as they can. Their goal is NOT your health or the IAQ of your home - that is up to you. Telling consumers that they'll also need to spend money on MUA if they buy hood X will negatively impact their sales (ignorant consumer will just go buy a hood from brand Y instead because it doesn't say you need MUA even though it has an identical need for MUA) so they do not want to do that. MUA is also not something that fits well within consumer hood manufacturers world except to the extent that they might offer a hood w/ integrated front curtain ducting. It is much more of a pure HVAC thing. It can be supplied to the return ducts of an HVAC system, the supply ducts or ducted directly to appropriate locations. Incoming air can be heated by gas, electric or hydronic. Incoming air may need to be humidified or dehumidified. It often needs to be electrically integrated with the HVAC system controllers to function properly. Consumer hood manufacturers - ARE NOT IAQ or HVAC people or engineers - they make something decorative that by nature must include air movement so they reluctantly include air movement in their product. Commercial hood manufacturers - ARE IAQ and HVAC people and engineers. Their customers are much more educated than consumers. The one singular purpose of the product they are selling is IAQ - so they design and sell systems that provide good IAQ. Aesthetics is quite secondary for them. Residential HVAC companies in the U.S. - ARE NOT IAQ people (they should be though). A tiny few, less than about 1%, know it well and a few more know it a very little but the vast majority know just enough, based on what they've been told at a 4 hr CEU course, to sound like they know what they're talking about but they don't really understand it. Most residential HVAC people do not understand air movement or why things are done the way they are - they only know how to use tables that tell them to do this or that and as soon as something is outside of defined parameters they're totally lost (though sometimes don't even realize that they're lost). Licensed Professional Engineers do (or most do, some don't) understand air movement and other elements. They are the people who create the tables that HVAC people use. They can think independently. Defined parameters for them are not tables but the physics of air - it's movement, temperature and components. Unfortunately, most U.S. engineers do not have a good understanding of human physiology (except perhaps for biomedical engineers). They do not, for instance, understand how CO2 functions in our bodies and how high levels of CO2 due to poor ventilation affect us. Or how poor ventilation resulting in high levels of VOC's, PM, Carcinogens and Pathogens affect us.