The most significant difference between the resin body on that link and the picture posted above is the front end.
The resin body being earlier has the front fenders parts of the body with an aluminum hood. The later trucks like in the picture above used a fiberglass single piece unit that lifts up in its entirety, exposing the entire front of the truck rather than just a hood opening.
From my perspective I liked working on the later trucks since the large front access the newer design offered to the engine area. My postal fleet had a bunch of them that we had to maintain. They had 6.5 liter non turbo diesel engines with 4L80e overdrive transmissions and 19.5 inch wheels and tires. The earlier trucks like the resin example used 6.2 liter non turbo Diesel engines with turbo 400 transmissions.
The one in the picture is a Grumman bodied truck. They used a GM chassis with Grumman's aluminum body atop it.
I can't tell from the pictures but it looks like the wheel bolt pattern is 5/10 lug. That 5 on the front and 10 on the rear. If so, it used a heavier chassis than the pickups did, with a solid beam front axle and leaf springs on all 4 corners.
If its an 8 lug truck, then it used normal SLA independent front suspension carried over from the trucks.
I offer this answer as a person who worked for the Postal Service for a long time doing fleet maintenance and repair on those trucks.