You wanna never worry about overheating? With AC in an early mustang?
2 core Alum rad (ACP makes one that looks stock) with a shroud and 6 blade fan.. OR add a SPAL 16in electric fan off a thermostatic switch.
The shroud helps immensely at idle, as does a fan that can actually move some air.
I've got a 331 stroker in my '65. Factory AC. Power Steering and a couple other things that just add heat. I've only had it out in 90 degree temps a couple of times.
My setup? ACP 2 core. Fab'd shroud with a SPAL 16in fan, driven off the holley sniper EFI (comes on at 190 and off at 180), aluminum GMP water pump and 180 T-stat (drilled). Temp gauge never goes past the middle mark, even idling forever with the AC on. (195)