You can FE5 a 2000+ LeSabre like mine, not sure how you could do an earlier version.
Find anti roll bars from a similar year Bonneville SSEI. They should help a lot. I'd do front and rear bars first, then if you need more, go with SSEI struts.
There really aren't any aftermarket solutions for H body cars as far as suspension upgrades. W Body cars get all the attention. So, you have to improvise. I was in the same boat (ha ha, get it?) when I got my 2004. Looked all over for improvements and found nothing. Stumbled on this site (back when it was Bonneville Forums) and found someone doing the FE5 swap on an SSEI. Figured I'd give it a shot, dug up the parts, and now the car corners like a slot car.
Only H Body FE5 I know of is the 04-05 Bonneville GXP setup. Which wont fit 99 and older H bodies without modifications.
2004 Buick LeSabre Custom FE5 GXP suspension conversion. 170Kmi
2001 Silverado 4WD 5.3L, 33" BFG ATs, Z71/Z82 pkg, 147kmi.