I'm not a purist, and I'd love to see one of these that looks good, but this one doesn't work for me. That rear window/back of cab is particularly awful looking. This is also the only El Camino-ized car that still has four doors, definitely an unusual design choice since they just open up into the bed. Guy also has a 61 Chrysler Windsor/Newport, and he does hemi conversions. 

https://phoenix.craigslist.org/wvl/cto/d/1957-plymouth-el-camino/6704353341.html

He says the Plymouth was altered when it was "young," presumably in the 60's, which makes me wonder if it was a flower car or just some person's failed attempt at a Mopar El Camino. I doubt it's a flower car though, those were generally of higher quality and were usually with luxury brands like Imperial or Chrysler. Can't speak for the guy's other cars.