This^^
The 6870 can make more power than your 109 block can handle.
IMO, with your engine you are building, a PTE Gen2 6466 DBB would spool INCREDIBLE, have a huge powerband, make you mid-high 800fwhp, and with that 66mm turbine wheel you would still be able to spin the motor high, which you will want to do with a 220/220 cam.
90% of Buick owners never fully utilize the power and capabilities of the turbos they own. Instead, they buy bigger-than-needed turbos that flow a lot of air but are not as crisp in the responsiveness, just so that they can say they have the "flavor of the week" turbo, when in reality if they used a more properly-paired turbo for their needs, they would be elated at the increase in the car's performance.
The 6466 can go mid 9's with ease. I have a friend running mid-9s with his 6466. I have another friend running mid-9s with his 6465. Point here is that a 64mm compressor wheel would be quite sufficient for a 9-second car with great throttle response.
May I please ask who spec'd or sold you that 220/220 cam?
How did they arrive at that duration if you haven't even spec'd your turbo OR converter?
Does that not seem suspicious to you?