Measure the front half of your stock drive line(521). The newer driveline you will need to use should be about 4 inches shorter. I also have done the tranny swap on my 521. The driveline I am using came out of a 1976 kingcab shortbed 620 if I remember correctly. Also make sure you also take the carrier bearing from the donor 620. Thats what I used , hope it helps.....