/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.

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The crossmember (and mount) can be used regardless of transmission-904, 727, four speed, all the same.
The 904 driveshaft can't be reused with a 727, the transmissions are different lengths. It might be a tough pill to swallow, but if you plan on replacing the rear end, now is the time- otherwise you'll just have to have yet another driveshaft made up again later.
You should be able to make the stock linkage work with the 727, and the speedo cable will just switch right over too.
Swap mounts are available from Schumacher or TransDapt
'67-72 A-Body Slant 6 to 273/318 or 340/360
https://www.hedman.com/product-detail/4695
or you can make your own using a standard A body 340 driver's side bracket and a truck LA passenger side bracket.