Kurt:
I've seen it [beefing up the footwell area] addressed two ways.
1. the way you've gone and added plate on inner well as well as a ladder frame of 1/2" bracing the curved piece you already have in your picture. [see pic below]
*still have to address cross piece by where your heels would / might be if using stock OEM pedal cluster.
** upgrading to Tilton dual MC with bias-bar and drop down pedal cluster [top hinged] eliminates the cross piece under you heel problem
2. build in an entire "footbox" off of the lower cage A pillar and two cross dash bars of the cage. This is what our friends that crashed the 993 cup ended up doing. Lots of pieces but very safe.