I would pull a motor and trans from a junkyard and rebuild that as you have time. The cost to rebuild is going to be close to the same.
There are some differences between the two if you go with a 360. The 318 is balanced internally and the 360 is balanced externally so the torque converter is different as well as a different flex plate.
The heads on the 360 flow a little better and respond to port matching and cleaning up the bowls really well with a cam swap.