Hi Martin,
the D2 pulley is pretty close anyway to the seat tank so the pulley part looks about right but the breather is the problem. I'm not sure how that was dealt with back in the day. But since a D2 set up is a bit cramped for someone of your stature, Jeff's suggestion of 3" channel is a good way to do. I did that on mine (there are probably some pictures on here somewhere) and also set the seat back in the rearmost position. You feel like you're sitting way up high but it definitely gives the legroom and will provide plenty of clearance for the pulley, and for any access that you might need under there.
You might need to look into some additional support under the fenders since the J-series has the pressed steel versions and sitting up on channels leads to a bit of sideways torquing on the mounts as you rumble around the paddock