You need to get under this car and look at the frame where the upper control arms bolt to the skinny metal. Simply follow the (2) rear upper control arms to where they bolt up and then meticulously visually inspect all of that metal. You are looking to see if it is twisted, torn, folded and/or bent.
The weakest link on the G Body frame is the frame rails to the rear bumper due to corrosion, but a close item for contention is the metal where the rear differential upper control arms mount.
I have 604 HP in my '78 and along with having tubular control arms with Derlin bushing on all 4 wheels (upper & lower) I also have a set of brackets that sister to the upper control arm mount and then follow the floor pan and bolt into a stronger part of the frame in front of the rear wheels.
I have seen several frames with catastrophic damage to this area of the frame. IF, by some small miracle you only need new upper or lower control arms/bushings, I HIGHLY recommend buying these brackets and installing them being you're running wheels that are so far out of range on a G-body that the stress/strain you're adding to the suspension, frame & brake system is ludicrous. They don't cost all that much money and will just about eliminat any possibility of you destroying this part of your frame from here on out.
Feel free to ask me anything else. I'm a certified master auto tech, have multiple degrees, owned my own shop for 28 years until autoimmune disease wrecked my life and I'm seriously into performance engineering.