Howdy Back:
FYI the 200 and 250 blocks are the same length. The difference is in the water pump/pulley system. Early 200s use the short pump/pulleys. Later 200/250s use the longer pump/pulley system.
Since the 250 first appeared in 1969, I don't know if a short shaft water pump is available for them. If so you will need the matching crank shaft pulley/damper.
The reccommend for an electric fan is only for clearance and convenience. it can be mounted either in front or in back of the radiator. There is also a slight performance advantage in that the electric fan only runs when needed for cooling.
Welcome to the Forum. Enjoy the journey.
Adios, David