3sp on the floor to 4sp

I'm looking at a 73 318 Duster with a 3sp on the floor - all original, never been modified. How hard would it be to put in a 4sp in it?? Thanks in advance!

For the original question...if its 3 spd auto..you got lots of work. if its 3 spd manual, it will be pretty easy, just need everything for a 4 spd is all.

Phil