IMHO, if there is a factory lap joint, you should put back in a lap joint if at all possible. Otherwise, butt weld.
Like MAUSS said, cut out all the rusted metal. Beyond that, only cut till it makes sense. In other words, the weld needs to be in the spot you feel most comfortable making it. Then cut the patch panel to fit.
Don't use flux core. Like MAUSS said, you will need an argon/CO2 bottle to do this.
Some guys like to use 0.023. Others swear by 0.035. I have tried it both ways and can't really tell a difference. Maybe just my poor welding skills.
I feel that any hole with clean edges 1/8" dia. or smaller can be plug welded. I bought a length of 3/4" copper tubing at Lowes. Cut to a length of approx. 8 - 12 inches. Smash the last 2 - 4 inches flat with a hammer. Use that to back up holes when filling with weld.
Any larger hole and I use a metal filler piece. Hold in place with masking tape on the back side while you tack. Larger pieces can have a piece of weld rod attached to the center to hold it while you tack. Magnets can also locate a part, but watch out for arc scatter.
Tack every one to two inches. Then weld from you tacks. Only go a short distance and jump around in order to keep the heat down and spread the heat around.
Use double upped cutting wheels on a die grinder to knock down the weld proud. Then use a 36 Roloc on an angle die grinder to finish it off. If at all possible, you want to slightly "crush" the weld using hammer on dolly to remove the shrink. I try to do it between the cutting wheels and the Roloc.
Check my journal for the cutting, welding, grinding I have been doing on a 41 Chevy. Weather is almost good enough to get back out there. But we do have snow in the forecast for tomorrow!