There are a lot of variables...unless the 54 cab is ready to paint and install then you are probably better off fixing the 56 cab. If the 56 cab has a good roof...that is, no rotted drip rails...then the other repairs are usually worth the time and $$$ to make. Floors, cab corners, and rockers are pretty much standard replacements when it comes to cab repairs...if that's all you need then it would probably be better to use you $500 to buy replacement panels as opposed to a slant cab. I am not a big fan of using panels off another truck for repairs...too many aftermarket panels available that are in new condition and ready to install.
Here's a picture of my 56 cab when I was replacing the floors/rocker...if this looks too scary for you then maybe you should take another course of action.