Adding weight in car

If you can, you shold make this plate detachable. This will allow you easier repairs/maintenance/detaching your car and also carrying heavy loads.

BTW "extremelly heavy" plate welded within frame can destroy it, even if the welds alone are made properly

I have never ran any extra weight in my HD sidecar unless I am loaded up or have someone in it.

Your car can be so heavy itself, that with some experience you don't need extra load.
My cargo tug's weight, thanks to its special triangulation, is reduced to 30kg itself. Just to be able to carry heavier loads. So the extra weight is attachable, not mounted permanently.