I'm thinking off tack welding the perches on at 5 degrees nose down, then drop the axle in place to get the car on it's wheels and taking the measurement from there, whilst checking the trans at the same time.
Mike

69 Dart GTS 440 mopar .590 cam, Edelbrock heads, 3200#
best et 6.45, 106.78, 10.14, 132.88 mph, 1.47 60ft
best 60ft 1.36