I haven't played with DeSoto Hemis but I did install a 56 354 Chrysler Hemi in my 57 Plymouth.

As far as I know the location and style of the lugs cast into the block that the motor mount cans (the part that bolts to the engine that the motor mount slips into) bolt to are the same between all the Chrysler, DeSoto, Dodge and Plymouth Polys and Hemis. Meaning that the 341 should drop right into to Plymouth using the stock 301 motor mount cans and mounts (or stock 341 cans and mounts IF your engine is from a 57 DeSoto) . At least that's the way it worked putting the Chrysler Hemi in my Plymouth.

Also as I recall the cans that bolt to the engine are unique side to side and it does make a difference whether they are bolted to the front side of the lugs or rear side.....so make sure you mark them or take pictures before you remove them from the 301. If that turns out that the mounts don't line up with the frame mounts, rather than try and modify the cans, I think it would be simpler to simply bolt the cans and mounts to the engine, and relocate the stock frame mounts to where they need to be. Modifying the cans can be a much bigger PIA than relocating the frame mounts.
Because to the steering linkage and cross member location you will need to use the 1957 DeSoto oil pan and pickup.

The other area that may cause issues is exhaust manifold to steering box interference, depending on the exhaust manifold you are using. Depending of ow much room you have you may need to use either custom headers or possibly 1957 345 drivers side manifold.