518 short shifts and no od

The rear gear selection maybe giving you a problem if you have steep gears like 4:10's 4:56's etc,it will up shift quicker than most stock 3:55 in most trucks.
If your not getting a down shift your pressure is to low(after reading the first post).
Raise the line pressure,if the line pressure is to low it will also have no overdrive.
If you drive it slow upon a 1-2 up shift it should drop down to 1st when you floor it,if it does not your t.v pressure is to LOW.
If you disconnect the t.v your upshift times should be different from when it connected,if does not change you have a problem in the valvebody like fishy said.
If the t.v makes no change to the transmission performance try another valvebody(without the shift kit)
The overdrive problem could also be a bad solenoid,you may have more than one problem.