You would be better off trading that 429 for a 460. The 429 has around 10:1 compression and will demand premium fuel to run without pinging. Most later model 460s have around 8.5 to 9.0 compression and will run just find on regular, plus the longer stroke gives more "umph" for pulling power. No mater what tranny you get those big block Fords never did get very good mileage. I have run over 700 HP through a C-6 automatic in a drag car and never broke a thing except the high dollar torq-converters. A C-6 with a competition shift kit will outshift any truck-type stick transmission and is much easier on the rest of your drive train.