I am not knocking the testing but really once you have the hang of it you know in an instant if the weld is any good or not. You should be in a position by the time you start welding the chassis to know if there is penetration or not and that just comes with heaps of practice. You need clear vision of whats going on and plenty of time with the torch in your hand. Judging your first attempts at mig you wont be long
Its also a big mistake to just roll stuff around on the bench, as soon as you are happy with that start on some positional stuff. The learning curve begins again with vertical, horizontal and overhead., all of which you will need at some point in the build.
Bob