Have an 89 Mustang GT since new with 110k on the clock 5 speed manual and a 9 out of 10 concerning looks.
The only thing I have done to this car was a water pump and a new clutch ( complete ) due to throw out bearing failure and not due to abuse.
I live in Florida so not many cold nights but very hot summers.
The biggest battery I can fit is a 650 cold crank amp which is now over 5 years old. Had it load tested and was 90 % at CC capacity.
I usually had to replace the battery every 3.5 to 4 years due to instant death and had to push start it. I bought one of those cheap
hydometers and every cell shows 100%. Battery is a wally-world brand and has out lasted other major brands.
Had a cold snap and at 28 degrees she spun over fast.