Compare a 1966 Corvette Frame to a 1966 Impala Frame.
1965-'67 Impala frame
1964-'67 Corvette Frame
Note the difference? The Corvette frame has more cross-members to stiffen the frame, and it is fully boxed (basically a fully formed rectangle). The Impala, and all other GM cars and trucks used an open C channel for the frame. This is because a C channel is cheaper than welding in plates to fully box the frame, and GM wants a softer ride provided by a frame bending.
The frame is as much a part of your car's suspension as the control arms and A-arms. GM used stamped steel open channel control arms and A-arms. Like the frame they can be boxed in with the addition of flat steel plate welded to box the part as this limits flexing. You can buy kits of pre-cut and formed sheet metal to box a frame on a Chevelle but not an Impala:
There are You Tube instructions on how to box in your A-arms and control arms for a Chevelle but no one talks about the Impala.
But everything that works on a Chevelle will work just as well with the larger dimensions of the Impala. This is called Hot Rodding.
You can buy parts from many companies that make parts to improve your handling (Hotchkiss, Global West, PST, and others); but no one sells one all encompassing kit to bolt on a better frame and suspension (though chassis builders such as Chris Alston and Art Morrison will sell you one for a modest fee of between $7,800 to $12,600).
As an engineer I can tell you that the stiffer the frame and stronger the suspension (less flex) the better the car will handle. It is all about keeping the tire planted flat on the pavement at all times. The bigger the tire patch on the ground the more control you have over the car as speed increases. It all has to due with the equation ½mv².