When I bought my 50 3600 I too noticed the cab had a slight "lean". Turns out the front cab mounts were rusted through and could no longer support the weight of the cab. The PO had jammed pieces of 2x6 lumber between the cab and frame to support it.
I ended up pulling the cab off and welding in new mounts.
Here's a pic:
http://i671.photobucket.com/albums/...0%20Chevy%20Truck%20Project/Pict0525.jpg