Yes Diodes only allow current flow in one direction but that does not prove that melted wires , blown fusible links ,smoke and smell is because the negative cable was attached to positive battery post . And Positive cable was attached to the negative battery post . Fact is Positive had to touch negative somewhere . OR Negative had to touch positive somewhere . even if it was a wrench touching the battery terminals .
Do you still have headlights? Time again for the good old trusty fusible link JPG and link: https://www.v8buick.com/index.php?threads/replacing-a-fusible-link.325644
I hope you got the fuse link fixed. Have the alternator tested when you do get it fixed. It is possible it survived but could be damaged. Even if it is charging the diodes may now be leaking causing a drain on the battery when the car is off. A good alternator shop can test it for you, but I don't think a parts store will be able to do that for you. The alternator will not tolerate reverse polarity unless the Pos and neg are insulated from the alt case, which your Delcotron is not.
