Oh it's definitely related to the size and shape of the T56. You can see that the T56 is quite a bit longer and the shift tower is further behind the crossmember, so it hits the area where the stock tunnel has already tapered down into the driveshaft tunnel when you slide it straight back. That's why the opening behind the shift tower had to be so long, as it tapers down too so the shift tower sticks out as the transmission slides back. If I'd made the tunnel boxier back there with a straight down drop off it could have been a little shorter.
I think the spec I saw for the TKX was 96 lbs dry, the T56 Magnum is 135. Although the A833 was like 120 lbs dry.
That's a good point, I hadn't seen that upper hoop. The end plates on the T56 crossmember definitely make it taller. But I like that profile for torsional rigidity too. Everything's a trade off.
The TKX wasn't out when I bought my T56, but honestly I'm super happy with the 6 speed, and filling a 3" gap or a 1/4" gap is pretty much the same process, once it goes beyond a single pass it ends up being the same amount of work.