The transmission is fine, but your K member has to go, as well as the dinky rearend, and you need a shorter driveshaft than the 904 had. Remember, a duster driveshaft is shorter than a dart, so having one made is the logical route. then, worry about the different yoke sizes between the driveshaft and rear end. then, do you have big enough brakes? Slant six darts mostly had nine inch brakes, not good even going up hill! If that is a 383 polara then there are more concerns as well. Exhaust, then The kickdown linkage, and torsion bar diameter. then, look for a sway bar. Without one, you will be all over the road. I hope I didn't discourage you too much. a small block is much easier, if that helps.