I was changing my oil on a 2000 ford e 350 and noticed a huge whole in the transmission. It looks like there should be a cap or plug there. I checked the tranny fluid while car was off to see if it had oil and it did. I need help to know what part is supposed to go there. Thank you

 

Not to worry !

That hole was placed there to allow one to drain the torque converter when changing the trans fluid. There is usually a thin rubber or plastic plug that stayed in the opening when pushed in place.
It would be good to keep it in place and probably only a dealership might have one.

 

The Ford part number for the plug is E9TZ-7N171-A ...There was a TSB addressing these plugs falling out. Clean the area and apply a small amount of RTV on the plug or around the hole and push on the plug.
When I drained my converters, you have to remove the front bell housing plate to even access the drain plug. The fluid just drains out from the opening which that plate covers. The plug can stay in place.

 

Some converters have the drain plug not on the vertical surface and access could be difficult . Found those with drain plug just above the hole easier to remove and install.

 

Converter drain plugs disapeared in 2003 and newer. Once the front plate is removed, the drain plug is easy to access including the OP's transmission..The rubber plug was even on the transmission bell housings with transmissions having no converter drain plug.

 

The OP with a 2000 year vehicle now knows there is a drain plug to drain the converter if he/she chooses.