I would assume you tried to contact Toms2005GTO??
I don't know for sure but did LSPWR do that tub job?
Any reason for not going to coilovers?
There are a couple of mini tub thread floating around that I would look and see what people are doing the talking and ask them.
This thread is done by LSPWR who seems to have done A LOT of mini tubs for people. Nebo's car does not look to have moved the spring perch and yet achieved a mini tub.
Unfortunalty for you...Nebo is running a rear coilover:
Note...Tom's2005GTO posted in that thread as well. but by the looks of it, seems like you could still use the factory location of the spring perch. I don't know where you would move it to since it is right outside the frame rail.