Lower Control Arms : UMI Performance, Inc.
I would not get carried away with poly, you have 30yo rubber, new rubber will firm things up a lot without the harshness and squeaks certainly going to use some poly I just would not go wild. Poly on the front swaybar will help a lot, front arms could certainly use new bushings and I would go something like Moog rubber on those.
Body bushings are going to be shot and options to replace them hard to come up with other than poly.
Far as airlift bags, if sized right they do not jack up the rear, they can simply stiffen and you can plumb them separate so you can put more pressure in the passenger side to keep it level at launch.
On springs, like I said the 94-96 Impala SS is the same basic chassis, there are spring choices and they will be stiffer since the 91-96 cars gained weight with the ABS, airbags, electronics. Just remember the Impala sat lower to start so a 2" lowering spring for it will likely lower your car more. I would start with springs rather than spindles because there is a larger selection of new lowering spring than standard height springs, and right now you have 30yo worn out mushy springs. If you liked the springs you had and wanted lower, then spindles would be perfect.
On the late model engines some areas they are cheap, others not so much, think it is a combination of if there is a supply of rotten out trucks vs. hotrodders looking for the engines.
Couple years ago I bought a complete 3/4ton 6.0l and a "Escalade 6.0l" shortblock for $1200 with wiring and computer. The Escalade motor came with flat top pistons and higher octane tuning for a few extra HP but was same cam and heads, intake etc. and typically throw a cam and valvespring in these and they are good to go, no costly rebuild. The computer will control a 4L60E which comes with a 3.06 first gear, then with torque converter lockup you can run 3200stall on a daily driver, and with overdrive almost anyone is OK with 3.73s, and many are even OK with 4.10s. That all adds up to a lot of torque at the tire. typical "musclecar" TH400 is a 2.48 first gear and without lockup and OD you are going to stay modest on stall and gear.
The new stuff is so potent the 2007 and up 5.3l basic truck engines have heads that flow as well as well as the old basic BBC heads did, with aluminum heads and plastic intakes they are pretty light too. Also understand HP rating system has undergone some changes since the 70s with a few updates that if applied to the old school motors would seriously depress advertised numbers.