My experience with two FWBs has been that the oem (FE1) rear bar is appreciably visibly really smaller dia. than the SS bar. On my first one I replaced the oem front with the SS front from my SS (which itself then got an F-body bar), and then replaced the stock rear (the notably skinnier one) with the SS'es rear bar (which then itself got an HO rear). I then 'added' the FWB rear bar back onto the SS rear bar for no less understeer than without it (still shoved in corners even with 255/50-17 - Bilstien - old sacked out SS coils) So, I then got another SS bar from a friend and doubled them up back there. Now THAT ran it much flatter = very nice. And yes, more oversteer.
For my current FWB I got an SS bar for the front and took the second rear SS bar off the other FWB for the back as I don't need the current driver quite as road-hugging. There's another FWB regular on here who saw about my doubles and decided to just combine the oem rear bar with an SS bar and he's pleased. Just a bunch of reference FWIW.
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Trailer hitch - less rear end shaking over bumps/potholes.
[best Buick brace ever made. Plus, it doubles up as a trailer hitch to boot.]
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Bilstein shocks - huge improvement front and rear - turning, bumps, potholes, dips, braking. Factory air shocks are not good for body lift/roll in turns.
[have them on all my cars, even my son's '88 MC SS. Except I thought I'd try the Monroe Severe Service (poor man's Bilsteins) on my current FWB. OK, but lil bit stiffer compression. Will. Be. Changing. To. Bilstein. All around. I also hear KYB mono-tube Gas-A-Just are good alternate and cheaper.]
Aftermarket rear lower control arms with poly (UMI, Metco, etc.) - less jiggling over washboards, more stable in turns. Rear UCAs are stock and new rubber.
[1st Gen Extended Metco on the garage queen SS, Hotchkis on the FWB. Strongly recommend aftermarket.]
Springs - front are still FE1, should be a little stiffer. Rear Moog cargo coils are too stiff--car is much flatter in turns, but oversteered until I put in the 30mm swaybar. Shoulda changed the swaybar first, then played with springs.
[~1/2 coil cut off oem fronts (1" drop and stiffer but no risk of bottoming out like happened when I cut almost 2 coils off the first FWB), and left the stock rear coils alone - more notes below]
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Air Lift 1000 air bags in rear - least useful thing I did. Not helpful with the cargo coils, so I keep them at minimum pressure. Maybe they'll be useful with some slightly softer non-progressive rear coils.
[Agreed Airlift is overkill with your cargo coils. But then again, I left the rear coils as stock and just added the Airlift 1000. Only a few lbs. but levels the rear back up (after ditching the terrible oem airshocks) plus adds attractive 1" rake]. And ten more lbs. carries the heaviest tongue wt. when towing]
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Then it was time for a new 2.5 steering box and 18" wheels/tires. I'm sure the car needs new body mounts (the originals must be shot by now) but it is a lot more fun to drive than it first was.
[Big Fat +1 on the 18" and even going 'Plus 1" Rubber' (of course needs a program). Running 255/50-18 gives 'nearly' the pleasant ride of tall stock -65 series sidewall plus much bigger contact patch.]
