You can measure boost at the manifold as well as boost before the intercooler to see if your intercooler is the problem. I run a F1A-94 through a 6R80 and made 983/826 on 24 psi. I run a custom 24"x6"x12.5" intercooler with 3.5" piping and have about a 1.5psi drop between the inlet and outlet at 24psi. I doubt you're getting that much belt slip with the HOB bracket setup, but it's possible.