The goto way I've used is, if you can't put a deep socket on the bottom 2 bolts of the 3rd member, it's a 9", if you can, it's an 8".
The 8" has longer flats on the top and bottom of the oval, whereas the 9" is more just oval.
https://www.summitracing.com/parts/fel-rds13270/overview/...