Here's another forum post from a member who appears to have had the same problem:
http://www.mightyram50.net/vbulletin...put-shaft-seal

From what I've been able to find, the MD951482 seal is smaller (1.575 x 2.244 x .5118 (40x57xx13mm)) than the National 710319 seal. Have you measured the bore of the trans tail/extension housing and the the diameter of the slip yoke? If you have those numbers, you should be able to find a seal that corresponds to those dimensions.