I have a Richmond 5-speed Road Race in my ERA FIA with a 3.08 rear-end. This tranny has a 1:1 final. I originally wanted the Richmond Super Street 5-speed with overdrive, but learned from ERA that the shifter doesn't really fit in a Cobra: they said that the installation would sit very high and look like something more appropriate for Herman Munster!
I had thought about a 2.88 rear for a little better cruising RPM, but Rick Lake cautioned me that the stud-axles were a little weaker with that set-up, so I settled on the 3.08. Haven't had it on the road yet, but Jim Holden (who has had several ERAs) had this combo in his FIA and said it was perfect.
Jeff