You are indeed correct that the GW one does not fit an HEI distrib. That was my experience and I *****ed high and low about it here. I had Gram Messer (Dolphan87) custom make one for me for $150 plus shipping. I had to reuse the GW brackets that mount to the shock towers and rod ends. If you were to purchase these separately, these might bring your cost closer to $200. It is expensive relative to the other ones out there but it has some benefits the regular ones don't have.
First, it is made from 1 inch MOA tubing whic makes it tronger than anything else out there. Second, it has adjustable rod ends so you can fit to any application. You can pull the towers closer together if needed (which is highly doubtful) or spread them farther apart (much more likely). I am using a 14 inch filter with an HEI offset.