why the hell would you helicoil it? you tap it before you put it in and your good to go.
basically the material you used for the spacer, make it longer, whatever it takes to go through the frame. then tap it all the way down.
now drill a hole though the frame weld it where you currently did, then weld the backside, now take a plug or whever you choose, weld over the hold in the backside, filling it, then sand it smooth.
Now you have the whole tube to keep it stronger and from twisting and ripping. and you dont need a long bolt because you only need to go through the shock and into the threaded section some, not all the way though since the threaded section will keep the strength
heres a little sketch