That sounds right to me. You see the chunk I cut off, it's about that long. I know it's between 1in and 1.5 inch, so 1 3/8 is probably good and it can be off by 1/8 too. yes, the Th350 and 700r4 are the same bolt pattern so they share the same Tcase adapter. All you need to worry about is getting the output shaft the correct length and use that spacer and you're good to go. It's very easy.