8 3/4 pinion angle

I just checked a 68-70 B body rear I have. With the perches level, the mating surface of the pumpkin is 5* UP.

As far as setting it up in the car, you need to account for axle wind. After applying torque you want the angles the same. If you put the rear in the car with the static angles providing the desired 0* difference, it will not be right under power. BTDT... My car vibrated like a cheap motel massage bed.

Different rear axle traction set ups require different pinion angles. All are pinion DOWN in relation to the trans output shaft.

Stock daily driver 0 to -2
SS spring -7 to -10
Caltracks ~-3
Ladder bars ~-1
4 link ~-2*

Every car is a little different and a 2* change can make a world of difference on a leaf spring drag car.

Since the OP is asking about perch orientation, the rest of the deal after the pumpkin mount surface is 5* down is moot.