Actually I believe the suburban wheelbase is slightly less than a longbed reg cab, by a couple of inches.
The info I found shows the sub to be 129.5" and I know the longbeds are 131.5"..
Wouldn't make a lot of difference but the rear tires might be visibly more to the front of the wheelwell if a longbed was mounted on a sub frame.
I believe the picture of the crewcab above may be on a reg cab lwb frame.
Similar to Cousin Willard,,