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.