What you can do with the trans on the bench (after cleaning the bellhousing) is to install the converter and pressure test the assembly.
Stick the driveshaft in the extension housing, plug the dipstick tube opening with a rubber plug and c clamp, plug the cooler line fittings then pressure test the trans using the vent tube, don’t go higher than 3 psi. If you hear anything look for an obvious leak. Otherwise spray soapy water all over behind the converter then release pressure and pull the converter. Look for bubbles.

Or make a plug for the pump and pressure test again - you should be a lemon to see where your leak is.

As for the gap between the flex plate and converter bolt pads that is by design. Nothing wrong with .050” gap once the converter is bottomed out. If you can catch a nail on the hub it’s time for a hub.