What would be fun around town without being insanely high on the interstate, coupled with a 4-speed toploader then?
Well...my take on it is that step 1 is to replace the transmission and then step 2 is to get a rear axle ratio that works well with the new tranny.
Here's was "logic" on it.
A C4 has a 1st gear ratio of 2.46:1
A 4R70W has a 1st gear ratio of 2.84:1
So...if you did not change your rear axle ratio, but just changed your transmission, you'd have an "effective" change of 1.154 X additional torque, in 1st gear, at the rear wheels. This, by itself, would be equal to you changing from your 2.79 to a 3.25 R&P (well...almost anyway).
And the overdriven gear is .7 on a 4R70W, which means a 30% reduction in highway RPM (it's actually more than that, because it has a lockup torque convertor unlike a C4 that doesn't).
So...you don't want that 30%, so you can back that down by changing your rear axle ratio.
Going to a 3.55 set at that point means 27% increase in RPM in all gears compared to the 2.79....giving back most of the 30%....but the lock up TC means 200 to 300 rpm less by itself.
MPG soars on the highway and the car now has 1.154 transmission increase X 1.27 rear axle increase = 1.468.
So....you now have 47% more effective gear off the line and BETTER (not worse) highway gear ratio.
That's how i looked at it anyway. I did my transmission conversion first and the 3.55's next. Then again, my c4 was pretty much toast...so that helped make the decision.