Hey Edison,
that's 3M panel bond adhesive.
I made a lip that fit flush against the flat side of the quarter, along the wheel opening, so I could glue the quarter on there.
This allowed me to avoid welding, and also to easily roll the wheel lip as it's only one layer of steel thick.
Worked out great.
The fit of the quarters to the doors was pretty good, but not perfect. The gap is mostly even, closing up a little in the middle, and at the top.
No major issues with them matching the body lines though.
Here's my plan for the initial (and temporary) look of the car:
Although the current look, with the black rear and grey sides looks pretty damn tough! I'd love to be able to drive it around like that for a while.
Cheers!