The 99 has the 3.5 Single O.C. with 24 valves. Couple guys on here who should jump in shortly can fill you in more,,,but I don't recall any butterfly problem with the SOHC 24 3.5. The Harmonic Balancer bolt is an issue on ALL Mitsus.
It will most likely have 4.2 diffs. Locker, I dunno.
Don't think any bounceys came in any after the 93 SR.
There's a 99 3.5 SOHC 24 valve here in SLO Co. that has "1999 CA Emmisions" tag under the hood, so it's for real,,,but it is a Gen 2 LS in all other respects- no blister fenders and it has 4.63 diffs.
I'd get a good look at a picture of a 99 before I drove a long ways to see it.

dave h.
'89 Raider V6 5spd;Aisin Hubs;; Gen2 LS: frt. brakes, rear coils;U.C.arms;R.trailing arms;idler arm; rear LSD axle w.disc brakes ;2 battery system for Dog's fan; relocated ECU; custom bumpers;J.Baker receiver;Conferr roofrack; t-bar crank.