The water pump assembly dimensions listed in the Tiger factory shop manual are not correct. The 260 aluminum pump is listed as 5.15″, but is actually 4.875″. The 289 cast iron pump is listed as 5.43″, but is actually 5.100″.
The dimension is measured from the front of the fan pulley hub to the gasket surface. The rear cover plate and gasket must be removed from the cast iron pump.
When replacing the water pump, be sure to use a hub that fits the water pump pulley and fan you are using. The stock pulley and fan use a hub with a one-inch pilot. If not available, take a stock Ford hub and have it machined, then reinstall on a new pump with the correct assembly dimension.
Be sure to follow the precautions outlined in the shop manual when instilling the new hub, as it is easy to destroy the seal and bearing of the new pump.
If you want to use a cast pump instead of aluminum, or vice-versa, use the appropriate timing cover for the pump you are using. Either will fit all 260-289-302 motors.