I finished grinding all of the sharp edges and weld spatter off the bottom of the chassis and then welded in the floorboards from the bottom.
I put the chassis back on the dolly and then added additional welds on top of the floorboards.
After dialing in the settings on the welder, I’m getting much more consistent welds. It’s still a little tricky to get sufficient penetration on the thick tubing without blowing through the thin plate, but using a zig-zag pattern and spending 60-70% of the weld time on the thicker stock works pretty well to get even penetration on both pieces.