how much horsepower can the 904 take?

I found this article online: 904 Torqueflite Transmission - New Old School - Mopar Muscle Magazine
The article indicates that a pre 1966 torqueflite can be push started. Now, I have push started plenty of standard transmission cars by popping the clutch. So, explain to me how this is done without a clutch???
Rear pump driven by the output shaft, and getting a custom converter is getting tougher for the small-hub early A's, which also had smaller input shafts on the 904's then.
Anyway, push the car in neutral then push the (edit) low button, the system is pressurized by that rear pump anytime the car is moving,........unless You grenaded a u-joint!!!