lookforjoe
True Classic
I'm assuming nearly all the stress on the rear subframe mount, thus also the rear suspension pickup points, is down under acceleration. The suspension itself handles lateral loads in corners (thus left/right) as well as pushing the car forward. Those pickup points at the rear clearly are handling lateral load mostly where the front points of the rear suspension arm handle the forward/aft load.
So for the rear I attempted to transfer the load from the pickup points up to trunk floor crossmember, which as you know stock is not really that strong. In the early cars without the trunk access panel the load gets transferred to the upper crossmember. I wanted trunk access so added that into my car but made the access panel a load bearing member (as discussed on my chassis thread here https://xwebforums.com/forum/index.php?threads/project-overkill-chassis.34807/page-3#post-299818)
On the rear firewall I did something similar to what you are talking about, but again I used some 14G plate there on both sides along with a reinforcement where the tunnel attaches to the rear firewall. I didn't think to add more box section there since it is already a box section, just a little smaller box!
Looking at your pics, I assumed you were utilizing the covers for (potential) load transfer, they look great. I didn't catch the addtional plate reinforcements in the firewall work. I'm assuming the vertical plates were added prior to the horizontal sections.
I'm thinking I'm going to use box section across the top of the rear crossmember. On mine, the existing stock lip/flange that secures the access cover has a decent amount of rust, so I'll just cut that out, add a crossmember & add the lip back on top.
I'm curious how the right forward corner was dealt with beneath the surface plate, where the original inner vertical panel was removed. Since that area carries the right control arm mount, did you add any additional support or bracing in that 'well'?