If the seal has the following dimensions: Shaft 5" and Bore 6.25", then use SKF 49998. If it measures Shaft 4.5" and Bore 6.25", then use SKF 45150.
For other sizes you can cross reference any seal via
http://www2.chicago-rawhide.com/ > "Seals North America Part Lookup". Punching in Shaft and Bore diameters gives you a list of current seals.