From what I am gathering you are renting an apartment, so no shop or garage? Would you mind sharing what kind of welder, length of / type of extension cord (if any), and plug you are using? Those welds look great!
Just using a miller 140mig- on 110v and a beefy extension cord / hypertherm plasma cutter which has 240v plug. The miller is good up to 3/16 on gas with .o30 wire- 1/4" likes flux core or a second pass on gas. I had a dedicated 240 put in for the hypertherm.
No garage......fortunately I am close to 110 and 240 and big compressor.
The gaps will be filled with some 1" square tube which I seem to be out of. The old bacon bits sit 3/4" recessed into the tailgate frame and despite the curve of the 55s rear end.....it seemed to line up with the taillight sections with a little prodding.
Wow. This is really coming along. When I first saw this thread I was left scratching my head. As it's evolved, I'm starting to understand. Sometimes you have to think WAY outside the box to have a sane idea. Good job!!