The electric OD will engage at speed, so I would guess the manual OD could also be engaged at speed.
If I remember correctly, you were supposed to let off the throttle when you engaged the electric OD. Again, I'd do that with the cable operated model.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
