G'day Dave.
I'm actually wondering if the body is in fact an Arntz also. Wasn't the Arntz/Bennett/Butler renowned for having an engine that was way back in the chassis, almost mid engined?
Here's a pic I'd pinched from the gallery of "nevermind65" showing an Arntz engine bay.
If that was the case, then the pics you've posted as per your description have a firewall that is very far forward, unlike the majority of manufacturers out there. I'd imagine the donk would undoubtedly be well over the front axle line in the mystery kit.
Cheers.
Steve.