72 250 trans wont shift into 3rd
 

My 1972 250 auto trans wont shift into 3rd(high gear)...it does go through 1st and 2nd. Any ideas?
I have a 1971 250 that has a good trans. I was thinking of just swapping them if they are the same models. Do I just need to check the serial numbers on the trans body to tell me if they are the same?
Thank you for any help you can lend.