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???
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The old cast iron torquefites had a rear pump besides having the front pump.
Put it in gear and push it. The rear pump pumps, motor turns over.