‘73 Duster /6 to 318 Swap

I've done this swap a few times, and most recently in a '74 Duster. I've swapped K frames and used the Schumacher mounts.

• Mounts - You are correct, Schumacher mounts are the easiest, though swapping K frame isn't too bad. I used Schumacher on my current '74. I guess I'd do what you have easiest available.
• Transmission - If you go 727, the shift linkage you have will work, but the slant 6 kickdown will not. You have lots of options for kickdown including Lokar cables or even using easy to find 318 2bbl kickdown (yes, it will work with a 4bbl pretty easy). Keep in mind, if you go with the 727, you'll need a different drive shaft as the 727 is longer. You will also need a 727 torque converter. IF you used a V8 904, it would be a direct bolt in.
• other things to consider - radiator and exhaust?. Side note, the wiring harness and throttle cable will work from the slant to the V8.