I'll agree. You can watch the weld puddle as it becomes molten metal and is looking like a little melted dent as you move your wire/rod the molten dent will also move as your filing it in fusion to the other metal it is getting fused with. If one piece is thinner, mostly concentrate your main heat toward the thicker piece and again watch the molten metal weld puddle.
If you can not see a puddle and see a light where you are welding you need a lighter shade lens to see better. You must be able to see that molten weld puddle in the welded metal pieces or have a really good feel by sound from years of experience.
Don't even think about using a Flux-Core instead of gas. What a mess.
Practice, practice and practise with the same exact kind and thickness of metals you will be welding. Take your welded practice pieces and try to destroy them by prying, bending, hammering, air hammering, and finally cut them apart where you welded to check depth of penetration.
IF you were able to separate the welded pieces by any destructive method prior to actually cutting, that weld will not work in a race car and is not a good strong weld.
I am a decent welder. People bring stuff to get it welded to here including trailer hitches on ther race trailers, frame sections, structural members, brckets, ladder bars, 4-links etc. I do a decent job. Still I am not factory trained nor certified and would be scared to weld up a roll cage. Maybe it is just me.
Ed