It's the last game of the reg season. I'm playing a 103 win team and need the "W" to get in as a wild card. My staff "Ace", a LHP, is at 81%. I'm starting a RHP that I have some confidence in.
My question is: Do I use my Ace as a LHS and set him to pitch to only 1-2 hitters or do I use him as a long reliever and set him to throw an inning or two? His stamina is 74.

Set up a tandem and decide how quick you want his pull rating.  Might be your best bet.

egrant...Thanks for the advice but I don't think a tandem works. One I have confidence that my starter can get the job done if he's on his game, and two, I'm afraid for my Ace to pitch more than one inning, possibly two if he throws strikes and keeps his pitch count down...I guess my real question is how many pitches would you allow my Ace to throw? 10-15 maybe, or chance it and allow 20-25 pitches with him at 81%?

If it's a do-or-die game, I'd still go with the tandem. 

TPC for your starter (tandem-A) at 40, MPC at his regular number, and bump the pull rating up by 1 or even 2.  If you have another decent LR or SP available at close to 100% who you can somewhat trust, set him as tandem-B with similar MPC and TPC targets with a pull rating of 5.  Otherwise, put your ace in as tandem-B with a 40/40 PC.  If you don't tandem with your ace, put him in the 'pen as SuA with 10/15 PC, and everybody else as SuB with short leashes.

Either way, make all of your other pitchers available with short leashes (pull ratings).  In an elimination game, it's always "all hands on deck".  Unless you want to pull a Crump.

Well, he does say he has "some confidence" in his scheduled starter, which implies to me that he does not have "full confidence".  Which in turn leads to the idea that you still want to have him on a somewhat shorter leash in an elimination game than you would in say game #73 of the regular season.

But the last point about adjusting the ace's auto-rest is well made.

I guess it's a matter of whether you want your ace to be the first man out of the 'pen (Tandem B) or to pitch late (Set Up A).

The rest (aside from Auto-Rest... which I can't stress enough, as that likely cost me a WS once upon a time) is up to you.

pitch the 100% guy .... maybe you'll score some runs and it won't matter who pitches