If my math is right, about 8 tons it will start to stretch, so maybe half that.
Root diameter of 1"-8 thread is about 0.84", half that squared times pi is about .55, times 30,000 psi yield strength for grade 3 mild steel gets you just over half or about 16,000 lbs., aka 8 tons.
Fastenal doesn't agree, and thinks you can go higher, they list their plain steel threaded rod at 60000 tensile (breaking), looks like most of their yield strengths are maybe 80% of that, so maybe 12-15 tons reasonable working load per rod in your assembly, assuming even loading.
https://www.fastenal.com/product/fasteners/rods-and-studs/threaded-rods/600173?categoryId=600173&level=3&query=all%2Bthread&isExpanded=true&Dia/Thread%20Size=1%22-8&showAll=Dia/Thread%20Size&productFamilyId=23392
So with a 20 ton jack, two threaded rods (I'd think double nuts at each end would be cheap insurance if you have room for them) and as much heat from the torch as you feel comfortable giving it, while smacking the end, might almost be enough to get you somewhere.
Just remember, pump side down on a hydraulic jack on its side...
