can you put a c6 in a 91 f150 4wd that has a a4od right now

 

Yes you can.

 

will one from a 2wd work or from a car? and how would you run the transfercase from a m50d or a e40d?

 

To bolt a transfer case to a 2wd c6 requires changing the output shaft and tailhouising to 4x4 units, which requires complete disassembly of the trans, so yes you "can". One from a car will work as long asit is the right bolt pattern on the bell housing. 300I6, 302, and 351W are all the same.

 

do the crossmembers have to be changed to run a c6 from a m50d or a e40d?

 

Not sure on the crossmember but you are going to have issues with the computer when going from an E4OD to A C6... maybe just a CEL, but I'm not sure.

 

but that would help me out getting rid of that with the computer because i want to change over from a 4.9 to my 5.0 crate motor. and having the parts truck would be easy