I had a 1968 Charger. Not an RT. The only mod I did was put a low res exhaust on it and what a difference it made. It was no Superbird, but for an otherwise stock 318 it would haul ass.
A friend got a gorgeous blue 68 Charger 440 RT for high school graduation. We were in Texas. Took it to California and back. What a car. Only got one ticket, a cheap 78 in a 70 north of Amarillo going down a long hill. $16.