I have a 65 Barracuda with a slant 6 - three on the tree want some transmission/engine advice

And also keep in mind that a late model ('68 and up) automatic will have a torque convertor that will not fit the early crank without remachining the flange or getting a custom-built TC (and no, the proper early torque convertor will not fit the later transmission, either- different splines), will require a new driveshaft, speedometer cable, later carburetor with proper kickdown linkage (and also note that the later carburetor, depending on year, may also require a switch to cable throttle linkage), and don't forget that you'll need to swap out the radiator or at least add some type of transmission cooler...
Even the early transmission (pushbutton, cable shift) will require locating all the correct pushbutton and shift cable/park cable bits and pieces- since they are not generally compatible with the majority of aftermarket floor shifters.
At that point, swapping in the A230 (trans tunnel and carpet, speedo, driveshaft) may actually be LESS work...?