That would most likely be 8.5. The Hyper 8 Pro doesn't have the aluminum plate below the center bulkheads, and the chassis has some inscription that I think might indicate the extension of it over the original Hyper 8 chassis.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz
Just the mount I think. I have a hyper ST, and the mount goes right on top of the plate. The only thing is the posts are recessed into the chassis plate on this, so IDK if the mount would be too tall....