I think, all 340’s came with 727’s originally, but could be wrong. My 68 340 did. You have the 727 then it is probably the way to go. Doesn’t take anything really exotic to make a 904 live at a pretty serious power level, but as stated above, your mileage may vary. Two more things you need to address with a 904 than with a 727. Pump drive and high gear clock spring, (if you are going to spin it a bit) Billet servo’s? probably good idea for both, quality steels clutches?, both. Billet lever? Really depends on the valve body/line pressure. Sprag? Rebuilt 904 is good for most street car apps. 727? You want the best one you can get. Billet drum? Not really needed in 904. Good quality bands for both. If you really search for the best price, not much more than a comparable 727. If you are going through the transmission, the cost difference to fix the most common issues isn’t much more than a totally stock rebuild for either transmission.