I like those pictures with body work and it seems that you are able to do what has to be done right.
I am afraid, however, that with the L 319 as a car transporter you will be overtaken by a turtle...
For your car transporter I think you need a chassis as a basic, not a self-supporting frame part from a bus or a panel truck.
Do you built a new frame or do you have one from a 319 or from another truck?
I have a car transporter based on a Mercedes series 310, type L 608 D with an OM 314 engine - and believe me, it is very slow when you put a car on it...
Which weight should the car have which you will transport with your L 319?
If you want please tell more about how you shape your plates. Which tools do you use to make them?
Kind regards to Denmark