32 bit ports on an AMD64 system

[Jim]

I want to compile some 32 bit ports on an AMD64 system. I know the GCC
has to receive the -m32 flag to compile the ports as 32 bit, but I
also want to change the install directory with 32 bit ports, I was
wondering which would be the most appropriate root given unix themes
and standard FreeBSD directory structure.

/usr/local/[same-as-before]-32 (i.e. [...]/bin32, [...]/lib32,
[...]/libexec32, etc)
/usr/32/
/usr/32bit/
/usr/FreeBSD32/
/usr/compat/32bit/
/usr/compat/FreeBSD32/


Also, would I be able to pass the appropriate flags through port
upgrade (or use environmental variables), or would I have to make
duplicates of the port directories and modify the makefiles?

Thanks,
-Jim Stapleton