/6 to 340 swap

I have recently aquired a 72 Duster with a /6 automatic. The engine is junk, so I'm pulling it and the trans to install a 340/727 combo. I understand that I need conversion mounts for the engine to bolt into place in the factory /6 K member. Can the transmission cross member be reused with the 727? I assume the trans mount will be different as well. Will the slant 6 torsion bars sustain the weight of the small block setup long enough for me to sort out any issues from the swap? I will be keeping the stock 7.25 rear in the car for a little while, until I can source a suitable replacment. Can I retain the stock driveshaft length if I simply swap the yoke? All of my other Mopars have 4 speeds so this is my first delve into the automatics and I know nothing about them.

duster 2.PNG