let's think about this for a second... you put a battery with 4 more volts in a car ment only for 12, 14.5 max with the motor running and the alt charging. now just thank god you didn't do that in a car with a computer controlled ANYTHING. remember voltage isn't the only thing that changes there is also amperage which prob fried your starter, def the solenoid a second time, good chance the alternator and regulator and possibly anything else that may have wires running to them.

now if you are having problems with batteries lasting in the first place you should be looking for that issue not putting a bigger battery in it to try and make it last. besides car batteries generally only last 3-6 years anyways. do you have an amp for a stereo in the car?

put the stock battery back in and hope for the best. anything that doesn't work at that point is fried. check your fuses (all of them for anything not working) and the switch for your fuel pump. if the amperage was too high it could have burned it up.

"its not the voltage that will kill a person its the amperage."

and where did you find a pre 94 mustang with a factory v6? i've never heard of that and i've owned several including an 86 SVO (2.3L Turbo fairly rare). From 79- 93 as far as i knew you could get a stang with either a natural aspirated 2.3L or the good ole 5.0L, unless it was a special car like the SVO's, cobra's, saleen's ... etc.