1977 Plymouth Stationwagon ( still kicking )

hi all.. i am here once again for help / advice..
the tranny on my wagon is on it's last leg..
in the mornings i have to put it in drive and wait for pressure to build..
and now it barely wants to change from 2nd to 3rd, unless i let off of the gas.
and if i want to down shift, or pass, i literally have to stomp it to the floor,,
gradual press just makes it go into limbo.somewhere in between rd and second..
and if i go in reverse, i have to let it sit til it slowly shifts into reverse. otherwise it shutters..
the fluid tends to leak out.. this happened many years ago, the converter had hair line crack in it,,and flex plate had cracks too, and it had other issues..
i am needing a rebuild again for sure..
this will be the 4th tranny since i owned it back in early 90's
someone told me i should put a 727 in it,, but i am not sure about all the mods i'd have to do to make it work, i am sure i'll have to change drive shaft.
i know the 727 will bolt up fine,, but what other technical details am i going to run into ? i am told the 727 is more heavy duty... the 904 is light weight.
and the Fury wagon is not a light weight vehicle.
has anyone done a 904 to 727 conversion ?
i need to know all details...