using u-joints to replace a rag joint bad idea with solid body mounts?

[tuff57]

i am looking into the flaming river u-joints for my 72 camaro and i have solid body mounts in the car i read in a few places that this wipes out the steering box easily cause there is no flex so a rag joint should be used..i called borgeson joints and they same thing but i called flaming river and they said itll work with the u-joints
can anyone give me any advice before i spend out the money?

thanks joe