Rear factory overload/auxiliary spring capacity

[F250 390 Ford]

I want to add the auxiliary overload springs to my 1970 F250 Heavy Duty Special. I recently bought a 1972 F250 Camper Special complete rust free frame that has the factory auxiliary spring setup. I can't find any info on how much weight capacity the auxiliary springs add. The door tag on my 1970 without the factory auxiliary rear springs says 6900#, and the door tag on the 1972 frame I just bought says 6900# and it has the overload/auxiliary springs. It almost seems like Ford did not rate the trucks for more weight carrying capacity with the overload/auxiliary spring option? Does anyone have info on this?

[cep62]

http://www.fordification.com/library/pi ... tegory/228
Will this help?, there's a lot more in the links from he home page.

[F250 390 Ford]

I tried looking at link you sent me. I still am not seeing the increase gvw the auxiliary spring adds