Looks like a fun, but challenging idea you're toying with. A few things to consider for your next pass:
Perhaps this wouldn't actually be a part of your plan, but it is how it's drawn. If I'm not mistaken, that rear tire looks to be OE Camaro Sized. Based on the specs I can find, even if it was running 275's it would still have ~53" between the tires. Even with side clearance for geometric wheel movement, I bet you could widen the whole thing at the tunnel by another couple inches. And with those ~29" diameter tires, the chassis would also need to get proportionally taller at the rear bulkhead as well, especially if wanting to run rear fenders of a traditional-ish looking build, resulting in the rear wheel centerline moving back a bit as well.
The frame doesn't come to a point at the top of the bulkhead that the axle sits below. It flattens out and that portion of the frame is actually a trapezoid with a very narrow top, but it moves the shock mount pivot an inch or two back from the seating face of the rear bulkhead.
The rear axle center line does not actually need to be directly below the upper shock mount pivot. In fact, as opposed to a live axle, it would typically be less than desirable to do so on an IRS, as the axle would tend to interfere with the shock.
Due to the way our bodies and seats are shaped, the tunnel can flare out a bit in the last few inches before it hits the bulkhead.
The CV's have to accommodate a lot more articulation on a stock Camaro than they would a Locost. This otherwise unused travel could be utilized to move the diff rear of the wheel centerline until the combined angularity at max Locost travel is similar to that of a stock Camaro. Along these same lines, the diff does not actually need to be parallel with the vehicle center line. A little 'twist' could help gain some clearance for the flange.
Keep the ideas flowing!