I think HFR has confirmed the PX is from a '76 TA. Definitely a replacement rear end.
The gear ratio will be the numbers stamped on the ring gear like the ones in my last picture. Just turn the axle until those numbers appear. Divide the big number by the little number and whala... gear ratio
example 42/11 = 3.73:1
As far as what gear ratio to use... That's a matter of opinions and type of use you will have. A lot of long highway trips.... 3.08:1. Cruising around town, maybe something a little lower. Drag racing... there's even more opinions on that one.
Keep in mind the 455HO is a torque monster, so a gear ratio to compliment the torque is something to think about too.
Standard gear ratio for AC/Auto is 3.08:1
Standard gear ratio for 4spd is 3.42:1
I think 3.23:1 is pretty good. It's right in between.
I went stock 3.42:1 for my 4spd car, and chose 3.73:1 for my automatic because I want it to jump like a rabbit
Oh... and great decision to keep her Lucy Blue!