im guessing this is for the 79? I believe my link bars are 26" or 28" i cant remember the guys from suicide doors just told me to measure from the center of the axle to the hanger mount and thats how most people figure bar length and the inside of my step notch was like 2 inches above the top of the frame
Then I would just figure out where you want to put the new mounts and measure, everyone does it a little different. The longer the bar the less fore/aft movement you have, but other than that there is no right or wrong bar length.