I had a response written and removed it.
I'm actually not sure and don't feel like consulting the rules again for the stupid fine points.
I'm Probably wrong but here is what I think is the case
Probeam if bolted only to the suspension is legal for stock.
Once you tie it into the crossmember you get bumped to BSP.
Might be worse as stated. Either way, Total B.S. as this gets you even with the Solstice's stock ZOk crossmember
From what I remember, you can not remove and replace the backbone.
Doing so definitely bumps you out of stock and I believe into FP