One important aspect to consider when installing the Grandby on a 6.5' bed is the camper's CG relative to the truck's. When you install the Grandby on the short bed the combined CG of the rig will shift markedly aft. Airbags will help eliminate or reduce the squat, but, nothing will help move the CG forward. Get the CG too far aft and handling will be compromised. Plus the wear on suspension and rear axle bearings will be more pronounced. Ideally, you want to keep your "weight and balance" within the truck manufacturer's design parameters.
Switching from a Fleet on an little old Toyota to a Granby on a crew cab 1500 Silverado. Planning to install air bags or a progressive spring leaf. The Fleet was a lot for the Toyota, hoping the Granby although 8 foot will not be too much for the ‘14 Silverado with the 6.5 foot bed.
As far as the OP's post...
Hawk or Grandby...
Do you need additional storage inside the camper or outside?
I like the idea of a Hawk flatbed on a 8' frame chassis cab. Building a storage compartment between the camper and back of the cab.
Like this one built by the folks at Mule Expedition Outfitters...
The type of cab is irrelevant unless overall wheelbase length is a factor. I personally prefer crew cabs, but going with the 8' flatbed makes for a very long wheel base with the above setup. I'd probably opt for a super cab with the above setup.
If you have your sights on a flatbed, and installing it on a 3/4 or 1 ton ton NEW truck, consider ordering or purchasing a chassis cab. On Ford, the chassis cab frame is several inches lower than on a pickup with the bed factory installed. Further, if you order a pickup with a bed and opt for the factory bed delete, it only saves you ~$400. And the top of your flatbed and overall height of the camper will several inches higher than if installed on the chassis cab. The ground clearance and departure angles are very similar.
We have 2 black labs and find the slide-in Hawk very, very small. One lab sleep on the dinette seat turned into a twin bed and the other sleeps in her bed on the floor (kind of cramped for her). I'm not sure if a slide-in Grandby would help since the floor design is basically the same as the Hawk. The only difference is the dinette is a little larger on the Grandby.
But, the floor plan of the flatbed Hawk or Grandby would really help reduce the close quarters when we're all forced inside the camper due to weather or simply time to sleep.