Perhaps nobody's answering because they don't feel you're looking hard enough. In about a minute with "Search CC", I came up with this
Big Block Arntz butler for sale
and this
Arnst with no engine or trans in San Carlos, CA
and I'm sure there's more if you dig. Try looking under "Butler", also, as I think Butler took over Arntz at some point.
About three or four years ago, I stopped to look at an Arntz with a SBC at a dealer's. It was a nice 20 foot car, but did need some attention. They were asking $30K.
There are no Arntz/Butlers currently listed on Cobra Country, although it's a good place to look.
Hope this helps.
Lowell