Red, keeping it in overdrive will work, until you try and back up. The 1-way roller clutch, behind the planetary, will bind up, until it blows up. I can't imagine how these things could be too fragile. The guts are as big, or bigger, than the stuff found in a modern auto trans.
The guy at Bumper to Bumper in Beloit has one behind a pickup 4-speed, in an early 60's Ford. He says the only real concern is not to let the lube oil get low, or it will eat the carrier gears.