I agree that there was a reason that Chrysler put a 727 behind 340s, but even more iffy behind a strong and relatively stock 340 is an 8 1/4 rear end. Putting a 340 in front of a 904 and 8 1/4 is asking for breakage down the line if you push the car (which you must if you know the 318 isn't enough power at this point). Yes you can spend a bunch of bucks and improve the 904, but I bet a 727 and driveshaft would cost less.
I also agree with 318willrun, that 318 should run a lot better given what you have put into it. I would think that it would be a good investment to start a new thread about what is holding that 318 back and see if you can't wake it up so that you are happy and keep the 340 for when you wear the 318 out.
