I have made a bit of progress.
Butler website needs updating. This image of the full kit has the OLD bracket design that they don't sell anymore;
The second image is the new bracket;
I asked if they had a drawing or image of the new bracket, and they said no. I said they should update their kit photo. They did manage to find a few more photos from when they did the mock up, so I could at least see how they have the plate and spacers;
The plate and spacers is very similar to what GZ sell directly. I have also been talking to GZ Motorsports, and they sent me the CAD file for the bracket they would recommend for the Pontiac. I printed out a template for some test fitting. The normal low spot that lines up with 3 head bolts is too low and hits my suspension;
I'm looking at mounting it upside down and drilling an extra hole for the 3rd head bolt. There is a position which doesn't need an extra hole but it might be too high and the belt might hit the front of the waterpump pulley. Anyway, I think this bracket will be able to be made to work;
Unfortunately GZ don't have an adjustable mandrel. They have a fixed mandrel but it slides inside the harmonic balancer and uses a really long single bolt. I am not sure my Romac balancer is the same ID at that point as the stock one so I am not confident that will work (also not keep on the mounted method vs the 4 bolt mounts of the adjustable mandrels). I am thinking I will get an adjustable mandrel from either Butler (steel, cheaper) or Tin Indian (aluminium, more expensive).