69 Coupe T56 Conversion

[bevisbeck2]

I have a 69 coupe 351 w and a 4 speed top loader rear end geared at 3.89.1 and the 4 speed is killing me on the highway im wanting to swap out the transmission but im torn between the tko 600 and the t 56 id rather the t 56 because I wanna drive it on the highway and want the 2 overdrive gears but im not sure what all will be needed to do the conversion and I also wanna know if ill be able to buy a used t 56 from a donor car and it work with my setup because buying a new one is very pricey any input on this will be very helpful