This is nothing that we don't all already know, but at the moment I have 4 Mustangs in the garage, well 3, one is outside.

1965 Coupe 200 in-line 6 with C4 - Beautiful car, complete redo with no upgrades, I'm adding disc brakes and AC.
1966 Coupe 289 C-Code C4 - It was originally a 3-speed but the owner is now approaching 80 and was tired of the clutch, but no mechanical upgrades.
1966 Coupe 289 A-code C4 GT - No upgrades.
1968 Coupe (mine) 331 5-speed, narrow 8.8 rear, every mechanical nut and bolt has been modified.

The first three are friends cars that I service and maintain, and I typically only change fluids, check timing, and carb settings each spring, and they drive the tires off of them with tons of enjoyment behind the wheel.
On the other hand I seem to be under the hood or on the floor below my 68 chasing one issue or another every time I turn around. The first three don't have any drive-line vibrations, cooling issues, exhaust fitment issues, tire rubbing. Maybe I am my own worst enemy.

I just took the 66 A-code out for a drive and really enjoyed it. I have a line on a 66 C-code with a 4-speed,

Have a great day and enjoy your cars