U-bolt torque for Freightliner Airliner 2 suspension

I want to know if anyone knows the MAXIMUM torque for these u-bolts. They are 7/8" x 14 tpi (maybe 16). i just put in new spring bushing and want to torque as high as possible without thread stretch so the spring doesn't towards the center of the truck, misaligning the airbag and axle. This a common problem with old Centuries and FLDs. My manuals say 450-550 ft-lb. But I've heard you want to torque more on on this suspension. Thanks in advance

 

All the above done. I pulled spring out to push out old and push in new bushing on a shop press. Cleaned off rust with heat and wire wheel on an angle grinder. Frame still supported-had to raise to remove air bag. Do you know what grade metal the bolts are? I assume 10 grade.

 

Well, I removed 1 sping 2 years ago because it kept shifting. I cleaned off all rust, filled gaps left behind from corrosion with JB Weld, and smoothed out. I retorted the u-bolts twice after and it hasn't moved that I can see. I am replacing 3 other bushings, and cleaning up springs at the same time, but no JB Weld this time so far because the corrosion isn't near as bad. But I do maybe want to push beyond the torque limit when no JB Weld. Just wondering if anyone has torqued to say 700-800 ft-lb and cured the spring shift. Obviously I worry about thread stretch and being counter productive.