The two square slots in the converter hub engage corresponding studs on the inside diameter of a gear-type pump at the front of the transmission. Those lugs are pretty beefy, but I have seen them sheared off at times, resulting in no pump pressure, and sometimes some scoring damage to the pump housing from the broken-off debris. I always fabricate a small flat metal tab that bolts to the transmission case to hold the converter back snug into the pump while I'm installing the transmission, then remove it when it's time to bolt the converter to the flex plate. The tip about shimming the converter/flex plate junction is a good one, as long as you don't overdo it. A little fore/aft movement is necessary.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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