1962 Steering gearbox

[Phoenix SR]

Looking for some help. As we know there are 2 different gearboxes for our Bullet Birds, unfortunately the car I bought didn't have one. Other than the mounting flange spacing and the gearbox to steering shaft coupling style, are there other differences between them? Looking at the car, there are mounting holes for both/either unit(s), it seems that they could be interchangeable. I can see where my original unit mounted, and I have a replacement though it has the other flange mount pattern. Thanks in advance!