If it were me, I'd find a way to drill through/slot the stiffener to allow the bolt to pass through into the original nutserts. I would NOT drill and tap the stiffener because then the crossmember is only supported by the threads tapped into the stiffener. I'm not 100% sure but I think the nutserts used are a higher grade than plain grade 2 nuts so the threads are stronger than simple cold rolled steel. Also, by retaining the original mounting system the stiffeners will provide added support to the material around the nutsert so that there is less chance of deformation around the nutsert.

Secondly, if you're going to slot the stiffeners, radius the inside corners of the slot, don't leave a sharp inside corner. Sharp inside corners are stress risers. Radiusing them will help alleviate the stress.

Thirdly, do this before you weld. Drilling through welds is not fun. If you have access to air, get or borrow a die grinder and use a double cut carbide burr. They come in various shapes and sizes and will make short work of cold rolled steel.

Fourthly, this is by no means the ultimate answer. It's just the way *I* think about these sorts of things. So ultimately, it's your call.