One might get close with a narrowed El Camino cab back and the wheelwells and engine hatch from a Type II. Type III engine for a lower bed height.
I'm sure there's been a stepside old Beetle. Here's some
material from Spectre BGW's website (We swear we're still in business! [served from a dusty closet somewhere])
I'd go with the business coupe for legroom.
Was the one you saw yellow?
similar to VW conversions from the 1940s.
Drop a pickup box in the back of this business coupe