I expect that which port is input and which is output would depend if you want upflow or downflow.
A purpose-made contact/retention tank would have a blow down drain at the bottom for the things that settle out, such as rust particles. With a re-purposed softener tank, what happens to the solids? I am thinking you might be better to have downflow so that the dip tube could suck up solids and feed that to a mechanical filtering stage. But don't know. I have no experience in this. I am just trying to think it through.
If you were to clean out the tank as needed, maybe upflow would be better to retain the solids for the cleanout. So maybe every year or five, you would tip the tank and wash out with a pressure washer? Or would you periodically pass a high flow through to try to wash out the solids without opening the tank?