the srod-4 has a 17% overdrive. having the c-4 with that extra gear between low and high would help performance and you can switch to a little newer rearend with taller gears to help with cruise rpm. the ford falcon 6 cyl performance handbook lists some codes and rearend ratios for the first and second gen. years. it refers the stock rear as 7.5" though. they are actually 6.75" ring gear size. | 
