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

If she can drive a manual transmission, she can learn 3 on the tree. I bet she will end up liking it, I know I fell in love with it myself.

Your easiest and most cost efficient option would be to swap in the 904 automatic. No modifications to the sheet metal necessary like a 4 or 5 speed.
I personally would not swap a 273 in place of the 225. If you are going through the cost and effort of the motor swap, put a 360 in it.

I would upgrade to a (new)2bbl carburetor on the 225, add a 2.25” exhaust, and dial in the ignition system. Rebuild/buy a 904, aftermarket floor shifter,and spend some money on a torque converter with a 2800 rpm stall.

Even with a stock cam and 1 bbl carb, a modern high stall torque converter would make a noticeable improvement.





But really you should big block swap it.