The transmission is fine, but your K member has to go you can use swap mounts, as well as the dinky rearend you can use it til you swap to a 8.25 or 7.25, 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! The 9" drums aren't that bad 10" ones are much better 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 exhaust isn't to terrible, torsion bars are okay I know of many guys running BBs on S6 bars and the kick down isn't a problem use a floor shifter. Without one, you will be all over the road a sway bar isn't 100% needed but can help. I hope I didn't discourage you too much. a small block is much easier, if that helps.