Low output at idle w/ac on?

I know I’ll probably get some crap from some of you guys, but this is about my fifth Avenue and not A Body!

I believe it has a factory 65 amp alternator I could be wrong, but it is the original alternator put on in 1987. This car has 40,000 original miles but had been sitting since 2003 when the original owner passed away.

I’ve mechanically gone through the entire car and the electrical system checked out fine a year ago when I first started daily driving the vehicle now in the last two months while driving with the AC on or any other accessories playing at a stoplight the voltage drops down below 11.
While driving, it’s putting out 13 to 13.5 V.
I guess my main question is is there an upgraded alternator that will bolt on? I know I’ve used high output Police spec alternators in the past and smaller Denzo units on engine swap cars over the years, looking for quick easy over the shelf and reliable with high output at idle!

Thanks for helping in advance!