Yes, they are very different and use completely different motor mounts. But the blocks are the same.

I believe that the early 326 used a different balancer design than the 400 and the all later (68 and up) motors. That may be why there's less clearance and no notch. I have a 67 and 69 subframe I can check out later to compare the two, but I don't have a 326 balancer to look at to see if I am correct. Perhaps one of the other guys here with more than one car can compare for you.