I'm not sure of your resources , so I'll comment as if a younger version of my team had designed this. Next time, it would help us to know if you have a full CNC shop at your disposal or two students with a multitool.
I've come to really dislike multi-level base frames, because they often add a lot of weight without a lot of functionality. There's really no reason why you need a second level of frame here, especially made out of 80-20, which is quite heavy. If you want the flexibility in design that 80-20 provides, you can connect sections of it directly to the AM base, by bolting through holes in the c-channel to biscuits in the bottom of the 80-20. If you're worrying about strength, the AM base is quite strong, especially with a bellypan, so there's no need to strengthen it a lot.
I hope you've considered the benefits (and costs) of going dead axles. I'm a huge fan of live, sliding axles, but I do accept that some teams like to go dead. Don't just jump to the conclusion that one or the other is better, think about the advantages and disadvantages in the context of your team.
In regards to bracing the transmissions, those two lower holes on the gearboxes look ripe to attach to two of
these, like others have suggested.