i have a 74 plymouth duster with a 318 3spd on the floor,
but my tranny is shot and some friends of mine hav automatic trannys.
does anyone know what auto tranny will be the easiest to put in my duster?

 

The 727 from a small block or the 904 from a small block will fit with no issues.Be carefull to use the correct torque converter,one from a 360 will not work with your 318,a 340 converter will work fine(68 to 72) and provide a bit more stall than a stock 318 converter. As for overdrive trannys,try to find an a-500 from a v6 Dakota in the early 90,s range,it is a direct bolt up.The A-518 will require firewall modification to fit.

 

I'm not 100% sure, but I believe the OD transmissions will also require trans crossmember modifications. Magnum 360's will have weight on the converter or flexplate. Less than a standard 360 but more than a late 340.

If you get a small block 727 or 904, look to see if it's a lock-up converter version. Those started in the late '70's and can be identified by a smooth, machined tip on the input shaft. Standard versions have splines all the way to the tip. You can't mix transmissions & converters between lock-up and standard versions. Lock-up trans./converters are not performance oriented.

360 transmissions will have weight welded on the converter for the externally balanced 360 engines. You'll have to knock/cut off the weights or get a neutral balanced converter for your 318. Most '72 and all '73 340 transmissions also had weight on the converters, just less.

It might be easier to use an aftermarket floor shifter with the trans rather than finding all the parts that will fit. Either way, you will need to get the kickdown linkage from the carb to the trans.

727 and 904 transmissions are different lengths and use a different sized slip-spline yoke on the driveshaft.. I don't know how they compare to the 3-speed. If you use a different driveshaft, you'll also have to make sure the rear driveshaft u-joint fits your rear end. I believe there are 2 different sizes - 7260 and 7290.

Don't forget the trans cooler lines. You can use a remote cooler if you don't want to change radiators to one for an automatic.

 

It would be much less hassle to either put in a 4 speed or just rebuild the 3 speed. They are not very difficult to do. You dont say what is wrong with yours, it might be as simple as a broken shift fork. Passon Performance is a Mopar 4 speed rebuilder and parts supplier, google them and contact them about parts for the 3 speed.

 

Locomotion is right. Just knock the weights off of the convertor from the 360 tranny and it will work fine with a 318. An air chisel works well.