I was thinking about buying this truck and noticed this cover underneith is missing. The torque converter and flywheel is hanging out in the open. Im pretty sure it a turbo 400 trans. Its a older truck '80s 3/4 ton. Could dirt have gotten in somwhere and caused damage? Is this missing a big problem?

most likely there is no damage to the flywheel but i would suggest to find a cover to put on so no rocks or debris get inside and do damage, check your flywheel for missing teeth also

I have seen a lot of trucks with those missing. Many had stripped out bolt holes. I would replace it just to keep mud and crap out.

That cover also adds strength to the rest of the transmission case. Just replace the cover and also the strut rods that go to the motor mounts.;)