Welcome. Aftermarket parts are few and far between for these cars so pretty much everything you do is custom or retrofitted.
Anyway, the easiest way to boost power in a hurry for fairly cheap is to open up the exhaust and the intake.
Intake: pull the stock air box, install a K&N cone filter on a tube, encase in a CAI box to keep out hot engine air.
Exhuast: Replace stock cat with a hi-flo cat converter (dont gut it tho...), and get a hi-flow muffler like a Flowmaster 42583 (single in, dual out):
Suspension: Replace all the factory rubber sway bar bushings with poly bushings. Get a strut tower brace from a later model H-body from a junkyard (this is a bolt-on part, stabilizes the front end on corners:
http://www.lesabret.com/phpBB/viewtopic.php?t=4251 <<what it looks like and where it goes).
Brakes: You can pull a 4 wheel disc setup for your rear drums... this is a do-able project but difficult to get everything to work properly:
http://www.lesabret.com/phpBB/viewtopic.php?t=2331 and
http://www.lesabret.com/phpBB/viewtopic.php?t=1285
Anything else?
