when your down to 1200 rpm trying to pull a load if you go to shift your done.

Are you refering to downshifting when pulling a grade here? If so I say BS! If you are referring to upshifting, then yes, you would need to be in the 15-1600 RPM range to shift to the next gear.  I've not built ''probably over 200 60 series Detroits, but I have driven probably over 20, and my best fuel economy and pulling performance was running in the 12-1600 RPM range, with flat-land cruise set at @1300 RPM and @ 68 MPH. Go to the online Detroit charts that show the torque curve, torque increases dramatically from 1800 to 1200 RPM where it peaks.