For Q1 - the STOCK Differential only has a fill plug, on the top RH side facing the Pinion. Square drive, use the end of a 3/8" extension. I suggest remove the 10 bolts off the cover, and remove it to drain the fluid...
A shame no one out there provided you input.
Still interested?
I have had my 77 with the 400ci Pontiac motor since I ordered it 45 years ago, and I know a thing or two on these motors and about everything else on the car.
Unfortunately, the 400 - 455 Pontiac with the rope seal leak can be due to crankcase pressure.
Even with a correct PVC. And especially if the motor is modified for more HP/Torque.
A Viton-based main seal (Butler) will help in most cases...