!991 Olds 88...can I replace with a shorter serpentine belt?

Did the belt jump off track or break ?

the ac is an idler pulley in the system so leaving it out will require a total change of how the belt is fitted and not just needing a shorter belt
remember that the ac pulley bearing can be replaced if faulty and if the ac compressor has failed disconnect the ac clutch wire and there is no need to worry about belts

The belt has most likely moved either in or out on the bottom pulley. Check it first. I had the same problem it looked ok but was out one groove missleading designed pulley.

problems include failing ac pulley bearings , failing compressor , excessive gas in the ac
have an ac specialist check the gas pressures
before replacing the belt see if the ac pulley can be turned by hand with the ac off and then on

the clutch and bearing assembly is failing it is a freewheeling pulley till clutch is engaged but it has bearings which can fail

i think the have a pulley to replace the ac, if not contact gates belts and go to their catalog

a bad belt tensioner leaving too much slack in belt causes it to break.

Some Parts Stores Have A/C Bypas Belts you should call around

More than likely it's the A/C compressor clutch is going bad. Cut the old serpentine belt off and install a shorter belt to bypass the A/C compressor for now.

You can buy an A/C bypass pulley from an O'reilly auto or anyone that supplies Dorman parts. the pulleys are cheap and allow you to mount things and use the stock serp belt. You will have no AC ,though. The kit is designed with the intent of eliminaing the compressor, which is very expensive to replace.