I have a small Lincoln 125 wire feed welder (20amp). �I used it on my entire restoration. �As Gun suggests, practice, practice, practice. �Every welder will be different, but on sheet metal, lower the amps and slow the wire feed way down. Use the stitch weld technique. �Small short tack welds. �Jump acros the patch. Tack. �Jump back to the other side. �Tack....And so on. �Be patient. �Don't rush it. �If you do, the panel will heat up too much and you will distort it.
On the thicker heavier steel, turn up the amps and speed up the wire a bit. �Penitration is key here. Pay attention to the puddle behind your wire, not the bright light. �You want to keep the speed of your movement just in front of the puddle. �Don't move so slow that the puddle catches you, but don't move so fast that you out run the puddle. �
Again, practice, practice, practice.
On the steel gauge, some areas are 16 gauge, some are 18. �On mine, some were so deteriorated, they may have been down in the 24 gauge range.
Bottom line, the sense of satisfaction you will have by doing the repairs yourself, will make a huge difference in the bond you will develop with your Willys. Although, sounds like you've had it for some time. Bond is probably already strong.
Have fun.
Tim