I can't help with the length of the bolt , but the one you have described sounds like an original - Mr Douglas made them from a stud with a welded on nut!! If your shock absorber is riding, it is probably down to a missing spacer or incorrect one. If you dismantle the shock absorber, the sprocket should turn on the splined sleeve and lift the cam past the end of the sleeve. If it doesn't lift that far, the cam/sprocket lobes are excessively worn. The cam should be capable of lifting beyond the end of the sleeve and there should be a spacer about 1/2" long between the end of the sleeve and the spring end plate - the spacer prevents the shock absorber from riding over the top.
Hope this helps,