The basic simple amswer is the cage tubing must be a minimum of 1 5/8" x .120" wall. Without knowing the engine you plan to use the other safety equipment required is difficult to itemize. There is no vintage body class for belly tanks, only vintage engine classes. You will racing against other tanks and rear engine dragster like confurations.
You can ask questions here, many build diaries available. The HAMB message board at
www.jalopyjournal.com is a good place to go. There are many traditional guys building tanks there too. On that message board are many European/Sweden and other country builders.
The Australian Belly Tank thread on this list will be of great help.
The 2009 SCTA rulebooks will availbale around March 1st. You can order a book, everyone will recommend several, from sales@scta-bni.org.
Good luck with you project, I hope to see you on the salt in 2010.
DW