Some say that installing the roof top vent is almost harder than insulation. They may be right.

I did the install by using these instructions which I have edited for clarity

Tips for the Roof Vent Adapter:
We recommend  Window-weld adhesive which you can get at Napa. 

>> Some use 3M 4200 as well. I think any quality caulk would work well.

Locate and cut the 14 x 14 hole. This is generally between the two frames that are in fact 14″ apart. You can use the adapter as a guide to mark the hole. Stay clear of roof supports, obviously. 
Make a tape line 1/4″ beyond the adapter perimeter. 
De-gloss the area with a Scotch- brite pad. No need to scuff. Just dull the paint. 
Clean with isopropyl alcohol and let dry thoroughly. Blow dryer if it’s wet and/or cold out. 
Apply three (generous) 1/8″ beads of the Window-weld on the face of the adapter. Then press the adapter down onto the roof until the adhesive oozes out the sides. (I was not generous enough on my application. It’s not necessary to clamp. If you do, use a lot of lightly set clamps and spread out the clamp force over the whole surface area. (I used12 lightweight spring clamps.) 
Using the adhesive that oozes out, make a fillet around the adapter with a double gloved finger. (Just in case one breaks). 
Pull the tape within 15 minutes. Cure is fast on outside. Let cure overnight and then proceed with the fan install. 
(I changed these steps to drill before the butyl tape. )
Drill two 9/64″ pilot holes through the adapter but stop before you cut into the roof.
Then use the 3/32″ drill bit to make holes that go all the way through the roof and any frames below.
Now use two screws on the sides (1 1/4″) to secure the flange and drill the rest of the holes.
Again use the 9/64″ to make pilot holes and then 3/32″ to drill through the metal.
Then use the 9/64″ bit from below to open up the 3/32″ holes where they penetrate the roof ribs. STOP before you drill through the actual roof. A stop bit, if you can find one, will work. 
and into the van roof. Only where you are into a roof support beam should you drill a clearance hole in the beam. You only want the screw to engage the adapter and outer skin. The screw attaches to the metal roof only and that hole has to be 3/32″
You will need longer screws, I used 1 1/4″ screws for the sides that are parallel to the sides of the van. The 1″ screws (provided with the Maxx Air 7000 fan) were fine for the front and back of the flange.

I used the recommended butyl tape for the fan install. I covered the fan flange with 1 1/8″ wide tape. Placed the flange onto the adapter and pressed down. 

Then tighten the screws very lightly against the fan flange. Some butyl will ooze out which you can trim off. 

After all screws are in coat the fan flange adapter and onto the roof about 1/2″ with brush on bed-liner or Flexseal from the TV guy works well. Some sealants will melt the fan plastic so you might try it on a portion of the interior bezel first. You’ll cut a lot of that piece off. Good idea to check the integrity and reapply the sealant every so often. 
Dicor self leveling sealant also works but looks a little messy.

Roof van flange installed on the Impact 3D Shim. I used 3/32″ to pre-drill the roof for the #10 screws
I used 9/64″ drill bit to clearance drill the Transit Roof machined shim from Impact 3D

Roof fan installed

 I had to ask for more screw from Maxair for the sides of the van ( 4 screws) that were supposed to attach the fan to the flange.