I have a 1965 Mustang 289 with a factory equivalent altenator. Will I have to upgrade the alt to put in a nice head unit? It currently has a Jensen CR225X which fits into the factory hole. I am going to leave that in there and put a good headunit the in glove box. I dont plan on running any amps, it would just be a headunit probably 50*4W peak max. The lights do dim with the AC, radio, and high beams on when I slow to idle but I wasnt sure how much more draw a good radio would have over the Jensen. (They would never be on at the same time)

 

Man I run a JVC head with a boss 400w amp powering 2 subs. I can see the instrument lights dim at idle when its raining outside, the wipers are on, the highbeams are on and the radio is blasting.

You should be fine.