Are you reinforcing the corners of the larger grate? I see potential problems with racking of the grate that is that large. The cross-braces under the grate will not be adequate to prevent corner racking. Neither will the welds. You might want to consider traingular gussets at the vertical supports, extending a few inches along the side, this would provide some resistance to twisting of the frame.
Considering the weight of the racks, especially the larger one, a 24" or 30" diameter wheel is a better choice, your leverage will just be better.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."