Assuming 8.5" fit in the guards, which they might if you don't lower it and roll the guards, you have a big negative offset of 1 3/8". Standard HK-HG rims are positive offset. So you will have to shorten it a fair bit.

I built the 9" for my HK GTS 3/4" shorter on both sides to fit 15x7" rims at 4.25" backspace (0.25" positive offset) and these rims sit with their centreline exactly where a HT 6" rims sits (so the rim sits 1/2" either side of the 6" rim). So the achieve the same thing you are looking for 1 3/8 + 1/4" = 1.625" shorter per side (2.375" total per side), and this assumes the tyres don't hit the guard, the inner guard or the leaf springs with this setup.

I'd be sticking to 7" or 8" with a 0 or small positive offset.

Plus you’ll have to watch what your Engineer will allow in offset and track change. For offset you are bound by the bearing source both for offset and rim width. I have to run either HQ-WB or Ford rear axle bearings to allow me to run 15x7 with 4 or 4.25” backspace, this is because both of these were standard from GMH or Ford with 7” rims with zero or small positive offset.

Edited by user Friday, 7 December 2018 10:13:38 AM(UTC)  | Reason: Not specified