727 in a 1966 dart wagon! HELP

So, as I thought a 727 or a 904 would be better. With either one I'm going to do a full rebuild and add performance part to handle the power. I've heard that a 904 uses less power and is quite a bit lighter. Any truth to this? Transmissions have never been my thing, I'm more of a motor guy.
Yes and yes; but again with the power level your working towards do you really want to worry about a small percentage difference, versus longevity?
But wait, what kindof back-end mods are you contemplating? Cuz if it don't hook well then does it really matter? But if you can make it stick, then it will matter very much.
You know when they blow up they can practically cut the car in half right? So you gotta run a blanket or something, and I doubt there's room in the tunnel for even the 904 and the blanket, So you might as well bite the bullet,open it up, and put the 727 in there. Then you can sleep at night.