1966 A904 loses all gears overnight

When reverse is selected, All regulation is supposed to be negated (except for the hi-pressure relief),and the pump is at maximum pressure. Ergo the only obvious points of pressure loss are the L/R servo, the Front clutch, and the manual valve.
If the pressure is weak the flexplate bolts are proved, and the tranny comes down. If the pressure is good, we know at least those items are OK and we move to the next test. I cannot think of a better pump test.
I suppose the regulator valve could be stuck, but A) if it was you would see it on the gauge, and B) if the trans has that much debris in it I'm rebuilding it. And of course if it was stuck, this would explain no output from the cooler line.
I suppose the hi-pressure relief could be stuck open.
I suppose the L/R band could be busted
I suppose the servo could be popped out
I suppose a lot of things
But my first go-to is the maximum pump pressure test.