If you're having problems getting penetration on sheet steel that is 18 gauge or less, you need to sell your welder.
If anything, the difficulty comes with having too much heat, and blowing through.
Butt welds, ground down to the original sheet thickness, are the right way. Period.
Lap joints are for those who are not skilled enough to get good edge gaps, or those who are too lazy to care. (production body shops)
Even the best in the business can get warpage when welding too.
How do you plan on using that hammer and dolly to finnish out a seam that is a double thickness?
For example, Ron Covell, Kent Smith, Ron Fournier are among a small handfull of metalshapers that are considered the best in the business, and highly respected in the street rodding industry. They all use butt welds for joining pannels.