Lets look at some facts
1. When you get GM ASE certified on Suspension in the 70's, the FIRST thing that's taught is that "Tires" are part of the suspension. Seemed simple back then....
2. RWD car suspensions are built for RIDE AND TRACTION...Even on Firebirds, handling is a last concern. IN Pontiac, "Drive- ability" is Job #1, and accounts for 90% of service work
3. Suspension Bushings, front and rear.
A. The factory rubber AND the poly bushings work on same principal in the Front and the Front part of the Rear leaf springs, a 15% "Twist" , plus the spring rate , is basically your stiffness, Which is a SLOW to respond suspension. ANY stronger springs will make this even slower, and a much harder ride.
B. Lowering the car Will require stiffer rate springs to avoid a bottom out condition, and with the Rubber OR Poly bushing, you have a much more Bumpy ride. Throw on wheels and tires with NO side wall and you have your 3/4 ton truck ride.
C. To accomplish ANY spring Increased rate one must make the suspension More responsive ( IE: FASTER)
D. Faster, and stock. The ONLY way to improve suspension reaction time ( aside from putting on gas shocks on a stock suspension) is to get RID of the "Bushings" and put in Bearings. Bearings allow your suspension do what it was meant to do, MOVE. Allowing this movement makes the wheel stay on the ground more, and also, with good shocks helps absorb a certain amount of the bumps.
When you convert to a bearing set up, your suspension comes Closer to a "Strut", which is where suspensions went after second gen's.
many cars I have seen with Polys also install them on the sway bar. MOST I have seen, the sway bars WOULD NOT MOVE if you disconnect the end links, because they are SO TIGHT.
Global West has some great videos to Explain the "Bearing" and how much it can help with a stock suspension. Here is a short video
Part number from Control am UPPERs is #1014 ( You do NOT have to buy their control arms assembled)
https://www.youtube.com/watch?v=dUQbcyVINL8