16" from the center of the shock mount to approximately the center of the axle tube. 15 1/2" from center of the shock mount to center of the bumpstop plate attached to frame.
Paul is right, you will have more options if you move the mount to the rear, but Paul and I were able to figure out a good replacement for a 3.5" lift. Between the two of us we can probably round up the part number for you.
Nick, from Nick's Trix has a stock '66 on his website with a Bilstein replacement as well. I believe that build is called Route 66 if I remember correctly. I contacted him about it before and he said to use the same as the front #24-065283.
Mike