Darrin:
For your project, I'd recommend the rectangular tube rear frame section. It's stand-alone stiffness is much greater than the round tube rear section which requires much more tubing to be integrated into the roll cage for equal strength. In addition, as a home-build project, construction, tying into the original chassis, and necessary sheetmetal work will be easier to do.
As for the 4-link choice, that could be debated for ever. We build and sell both equal and un-equal 4-link suspensions. Over the years we have had un-equal length cars have excellent success. Your choice will depend on class choice, expected power levels, transmission type and past experience or familiarity.
You can go to our web-site:
www.smithperformance.com
Our catalog shows both systems.
FJ