I've never heard of the JW Harris 20 ga. welding wire. I'm sure it was nothing special you can't already easily find.
Your best best is to convert the welder to gas if you haven't already done so. Then and only then will you have a welder that can do it's best job.
Welding 20 ga. is very difficult. Why make it harder on yourself than you have too? .030 wire or larger is too big for that light material. You need to be using .023 wire.
Practice on some extra 20 ga. you have laying around. Get your welder set up so it works well for you. You will never be able to do a solid weld on light material without warping. Tack welds are the only way to do. Even then you will probably get some warping. Move around the panel to distribute the heat.
This is where you need to work the metal with a hammer and dolly before all the welding is finished. This will give you the best end results. Practice, practice, practice. Make sure the seams are as tight as you can get them. You will easily have enough penetration even with tack welds. Check this on your practice pieces.
Be careful grinding the panels when you are finished. You can still easily warp a panel after the welding has been completed with too much heat from the grinder. Patience is key along with a lot of practice.