hi there
I use a clarke 135, its not the beefiest welder in the world but its more than capable for the haynes chassis. im not an expert by any means but i hope this helps
on 1.5mm i use it on 1 max to avoid over penetration and blowing through. Im not an expert by any means but i think your photos show a little over penetration( could someone verify this). With 1 max i tend to set the wire feed at somewhere between 7 and eight. 1 max also alows you to get good penetration while being able to take your time to drag the weld pool and get a nice pattern going.
On 2mm box i find the best setting to be 2 min at about 9 wire speed.
on 3mm plate or higher i tend to max it out and get the wire feed right up to 10, but you have to get that torch moving at a fair lick.
I hope this helps, the best thing to do is to keep practicing and eventually youll stumble on the perfect setting and youll know by the perfect "bacon" cooking noise and the look of oyur weld. A quick note- the small regulator on my 135 is crap and i find that the only way to get enough co2 to shield the weld is to have it set on full blast.
Heres a pic of what my little 135 can do, its not perfect by any means but its a good strong weld and looks ok, my welds were no where near this standard when i started my build.
Hope this helps
ps, its always easier to weld on sheet than to weld box section together, however you will have to weld loads of box on your chassis so i gues your better off practicing on box section
nathan