Hey mate,
I don't have any experience with the Hitch-n-Go product so I just took a look at the website (
http://www.hitchngo.com.au/why-a-frame.html)
"The mounting brackets, or mount kit, is model specific to the car being towed and bolts to the car frame. Though it's customized to fit certain cars, some adjustments may be necessary, such as cutting the bumper or drilling holes in the chassis. The mount kit comes with all the hardware necessary to bolt it to a secure place on the towed vehicle. Safety cables act as a backup mechanism in case the car becomes separated from the motor home. One end attaches to the car and the other to the motor home."
Seems the spacing of the A frame and mount are specific to model type. The engineering plate is used for NSW purposes where the towing vehicle, A frame assembly plus towed vehicle are inspected. I see no reason why the Hitch-N-Go can't be used by another Sierra owner, but just to be safe it might be worth a phone call to Hitch-N-Go to ask the question
Hope that helps,
Cheers
Colesy