Welcome!
You might want to look into getting an early inline-6 HEI.
Unlike the V8 HEI's (which started out with the coil-in-cap setup), the early 6-cyl ones (from around 1975, 76, and into 77 I think) had an external coil and therefore a lower profile cap.
Here's an example pic I found using a Google search ... one on the left is a 75 inline-6 external coil HEI and the one on the right is a V8 coil-in-cap HEI.
The external coil used with the early inline-6 HEI looks looks like this.
And if you can't find an early inline-6 HEI that's already set up for the external coil, I believe a later one can be converted by simply swapping to the early style cap & rotor and changing the wiring harness that connects the module to the coil.