I also suspect Portis influenced John Kennedy Toole because when I read Portis the dialogue reminds me of Confederacy of Dunces. And Portis was a southerner, born in Arkansas, so was probably familiar to Toole.
If you've never heard of him, it's a good time to start. Oh, and these novels are, to some degree, a product of their time, so some things are going to seem dated. But they're brilliantly written and laugh-out-loud funny.