Guys,
There is a thread on the R Forum about the America Roadster. I'm not sure it has been as helpful as the original poster may have wanted---so---Shmeless Plu Follows:
The story of the Aluminum sport Roadster (aka 'America Roadster') is well told in the Porsche Speedster Typ 540: Quintessential Sports Car. The story is told by Chuck Stoddard and is comprehensive. There is also a listing of each of these cars and the steel version.
The book is available from several sources, including, well, me.
Steve Heinrichs