Just a thought,
Assuming you have a complete 55-59 rearend that is in good working order, wouldn't it make more sence to just redo the perches on it and install that whole rearend rather than tear apart both rearends AND change the perches?
I believe those later rearends are a little wider, so if the rims/tires you are using are a little tight to the outside, then swaping the whole rearend may be an issue...
going from a 4:11 to a 3:90 is only going to get you about an extra 5 mph top speed. I have seen 3:36 gears available for the 55-59 rearends...
You will also need to change the tranny when going to an open driveline...
