Re: suspension upgrade
I just replaced a fair amount of suspension on my 97 grand marquis.
Back: I installed cargo coils, $70 at Summit Auto Racing in Ohio (google it). I have put these in a lot of big cars and have been very happy - they'll go a long way to shutting down body lean and oversteer in turns. Simple change too - cut the shock bolts and the rear stabilizer pins (I used a 10" metal blade on a recip saw - 10" for the top bolt - watch the gas line! - you'll need a couple) with a jack under the axle, slowly let the axle down watching the brake line, pop the spring out (check for tension 1st, it should be loose in the holder b4 removing), pop the new one in, install new shocks and pins and wala! I used KYB shocks but you could use "coil-overs" (shocks with an additional spring around them) for even stiffer suspension.
Front: I just changed the pins and shocks (KYB shocks again) but regret it - I should of used coilovers regardless for the front - now with the stiffer rear end the front seems even mushier (if that's possible!). Talk to Summit about the coilovers too, get the part # with your quote and then you can shop it, tho I've found summit reasonable (and if you ever get to the Cleveland area you have GOT to see their store - man heaven). Heck call Summit and ask them your question!
Good luck. If you go with the coilovers in the front let me know how they turn out.