Its been a long time since I restored a car w/8 track (I did a 74 SD 455, and a 70 340 Cuda, both w/factory 8 tracks, both in early 2000), and I needed belts for both. What I did was measure with a string on one, but eventually did a net search for 8 tracks. Eventually I found a 8 track forum site, and all I had to do was post the serial/model numbers on the 8 track units, and got what I needed. Sorry, I dont have the link, but do a 8 track resto search and you will find what you need