my 3 speed is jamming up

[mrmotorhead]

Hello, i have a 1976 pacer X with the 3 speed manual and it seems the trans keeps jamming up. The shifter keeps slipping between the shift rods and it seems it keeps jaming up. I was thinking about swapping in a 4 speed. how easy are the 4 speeds to find? do i have to rreally change anything to convert it to a 4 speed?
thanks
kyle in wisconsin