Alt. Replacement

The first issue is with the ammeter. Run it into the +40 range for any length of time and you have a mopar bonfire. The second issue is +35 yr old wiring ( lots of resistance with corrosion ) and it being too small to adequately handle higher amperage. Sure, it may last a few months or even a few years, but it's trouble waiting to happen.
You'd have to upgrade the wiring ( or you should ) for the 50 amp alt anyway. Run the 90/120 amp one. You'll be able to run an electric cooling fan and all kinds of other stuff.
Just take your time and do it right