When most people think rail dune buggies, they think VW powered machines
with similar vintage suspensions. Think about the street-performance possibilities
of a lightweight tubular steel frame motivated by a healthy V6 or V8 mounted
in one of these tubular steel engine cradles.
Another option is to retain the VW drivetrain, but upgrade the rear suspension using one of the engine cradle options - either the opposed coilover setup or rear coilover struts. The strut-style option is more easily adjusted for your choice in track width. VW IRS-style axles are required.