Anyone who tries to change the system will have to 'reckon with' me. = Anyone who tries to change the system will have me to 'reckon with'. [=I will oppose/fight anyone who tries to change the system]
She 'was reckoned' [='considered'] to be among the group's leaders. = She 'was reckoned' as one of the group's leaders.
When he won his first three matches, the other players realized that he was 'a force/man to be reckoned with'.
(..) Now, I like you two boys. You remind me of me. Seein' as how you two is from Arkansas, and we know some of the same people and we grew up in some of the same places, I reckon we can make a deal for somethin'. A deal for what? Food. Food for a boat. He's a bum, Ellis. Come on. (..)2012 Mud