Andy's got it right. I ended up with a 9" shock (AFCO 3890 specifically) on my 72 with Calvert split monos stock ride height bends. That left me with nearly 2" of compression at race weight and 2" of extension remaining without the shocks connected and the rear hanging off the ground.
I would take some mesurments before ordering your shocks to be sure.
72 Duster - 408ci W2, 10.18@131. Shooting for 10.0's in 2018!