1974 Duster 340 Engine and 727 Transmission Questions!!!

904 with good performance/race parts will be fine. A quality 7260 u-joint, like Spicer, will also work well. (I've run Spicer 7260 solid u-joints in my 360/904 race car for years without issues.) As mentioned earlier, 904 takes less HP to turn, so you'll be faster. (About .15 in the 1/4 based on '70's Chrysler testing between 727 & 904 - all else being equal.) A 904 also doesn't grenade the drum like a 727 when something goes wrong.

You will need a special B&M flexplate to mate the neutral balanced 318/904 converter to the externally balanced cast crank 340.

The short 727 likely is out of a truck or van.