There's a bunch of useful information regarding transmission/rearend combinations in the ERA assembly manual. Go here:
http://erareplicas.com/freeman/427.pdf
Scroll down to page 13.
I'm running the Richmond 5 speed with 2.88 gears. I think 1st gear is very usable (approximately 50 mph at 6000 rpms) and at 70 mph in 5th I'm turning 2700 rpms (just hitting 2500-6200 rpm window for my cam).
Chris