I have owned my fastback since 1992, having purchased it for $1,700 as a basket case.  It originally was a 289 "A" code 3-speed car and has had a 302, a 351w and is (in 2013) getting an all aluminum Dart big-bore 351w based 427 stroker with AFR 205 heads.  It has had the original 3-speed, the C-4 I installed when I bought it, a T-5 from 2002 to 2013 and is now getting a TKO600 five speed.  This involved raising the drive tunnel and making custom upper and lower crossmembers in order to make it fit at the proper angle.  The car has power windows and locks, 4 wheel COBRA power disc brakes, a N case 9" rear that has had 3.00, 3.89, 3.70 and now 3.50 gears in it.  All said and done, the 3.70 was the best gear, getting best performance and gas mileage.