Need compiler switch to set variables to 4-bytes in 64-bit build

I have some mixed C/Fortran code in VS2010.  I get different results from the 32-bit and 64-bit builds of the code.  To see if it is a precision problem, I would like to compile the 64-bit  build with a switch that forces the real and integer variables to be single precision (4-bytes).  Are there switches for the C and Fortran compilers to do this?  Please let me know.  Thank you.