Have the car weighed or weigh it your self for a total weight, then weigh the front end, then the rearend( both with you in the car). Once you have the figures then you'll need to figure out ride height you want to achieve for shock length...then it becomes a matter of whether you want a soft ride or performance ride which is dependent upon spring #rate/length for both front and rear.
I experimented with different spring rates on my Cobra roadster @ 2250 # overall weight, using QA-1 adjustable shocks front and rear, running a sbf . I finally went with 400# 8" fronts and 600# 8" rears. The car rode "hard" with this setup on the street but handled like a dream on the track....and gave the proper "look".
Hope this helps... once you know the weights, ride height a search on-line or contact with the tech dept. of the shock you plan on running will really help