I think you've been watching too much "True Detective", I don't get any kind of murder vibe in the song at all . To me the song is about a country girl who's trying to act all "citified". He calls her out on it but it is definitely still very attracted to her. Fiberglass has been used for insulation, boats, bathtubs and in this case cloth. I don't know why you immediately jump to caskets. I guess all religions are a bit different, I've never heard of a Sunday mass being combined with a funeral. Maybe he wants to go to Sunday Mass for confession or wants to feel pure again. If you follow the lyrics- he takes her to the river, wants to take her to New Orleans, and then ends with "March on baby, you're looking good". Why would he drown her in the river, take the body to New Orleans and then tell her to march on ??? All lyrics are subject to interpretation so you could be right.