Radek has it right. The kit was designed from the beginning to sit on a VW floor pan. Back in the day, people would buy a Beetle that had been in an accident, that damaged the body but none to the floorpan. Then take the Beetle body off and put the Eureka body on. Sounds simple, but often took years.
With the lighter body, a 1500 will push it along quite nicely. Fuel tanks probably vary car to car. As they often rust out when left empty for years, they occasionally need replacing. When being built, most people used the VW fuel tank as they typically had the original donor car sitting there!