I'd go with 6061 Finn, you guys put too much emphasis on the whole '61 vs. '75 debate sometimes I think. That choice is more important in other parts, not so much chassis material tho.
The 4 wheel steering might be a necessity to regain the steering, but I wonder if the best route would be to have a programmable servo that could have the rear EPA reduced. You probably remember Honda doing this with some production models, and they had the rear steer in proportion to the front. This would minimize the 'forklift' swing effect enough that your rear camber wouldn't be much of an issue. Just a theory, I am not to be held accountable if it's wrong
With the parts you have I wonder if maybe you could pull something other than a 'Savage' out of your bag of tricks? The savvy bulks are much more easily adaptable to different chassis types/layouts than say... LST diffs. So that's a big advantage. Not sure what to tell you about shocks, we need something sized between LST and Maxx shocks -diameter wise, for many of these custom builds, I haven't managed to find anything yet. The LST & Mug shocks were made more for the 'shock value' (yes, pun intended) than for any actual need for units that big. Marketing in my eyes, not much else. The main reason I opted out of those on my new build was weight/bulk/tallness (vulnerability to hits). And I believe the Savvy shocks are just foolishly long, making them more susceptible to damage. Never owned a Sav, so that's just my own ignorant pov on those.
You have options, no doubt about that!