I have a 518 non lockup trans swapped into an older truck. I use a switch to turn on o/d manually. It isn't tied into a brake switch or any pressure switch. With the OD switch in the "on" position, it will not go into OD until 3rd gear. However... If the switch is left in the "on" position, and the truck downshifts (as in coming to a stop) it will stay in OD through 1st, 2nd and 3rd.
Is this how this trans is/was functioning? Just curious as to if it can start in OD?
My trans is a factory built non lock up, so it may not act as yours does.

"use it 'till it breaks, replace as needed"