Interesting, are you doing this with the tires on? I built my truck using the 12 tons and have had both axles out a couple of times since without issue. I can put the stands behind the FUSH hanger and let the jack drop the axle(s) down on to a pair of HF car dollies. Maybe the difference is the custom drop hangers / shackles on the 1st gen?
No tires and the axle is on dollies. My truck is the same way (actually worse) with more droop in the springs.
However if you're putting the 12 tons under the flat belly of the frame (behind the FUSH) that might be the difference. With drivetrain hanging over the front that's too sketchy IMO, I prefer the stands at either end of the chassis for stability. Those 12 tons extended rock back and forth, fock that!
PS- I built both my trucks (more than once) with the 12 tons and a engine hoist. Now that I have quality jack stands I understand the safety I was sacrificing doing it the way I did in the past. In fact, I want a second pair of the sunnex stands now.