After sucessfully welding under my car for many hours it's definatly possible to get the job done with out a rotisserie.
Looking at you work, your welds look cold and dirty.
1. Make sure your welder is getting proper power. Skinny old extension cords and bad outlets will reduce the amount of current going to the welder.
With the proper power your welder should easily be able to burn holes right through 20-16 gauge metal.
2. Cleanliness is key. Make sure you remove all of the surrounding undercoating, paints, and seam sealers around the welding areas. Not only is that stuff extreemly toxic when its burns but it will contaminate the molten metal.
Although the welds for the floors and SFC don't look pretty i'm sure they'l hold up. However, I would consider redoing your transmission mount ( looks like a 25DET tranny). I sure wouldn't want contamination on the welds supporting a trans and 250+ Ft/Lbs.