I installed a Momo hub on my '74 a couple months ago. I don't have the car here to go look, but as I recall I had to do some minor mods to it to make it work.
From memory, my biggest issue was that the "tab" on the stock horn ring to which the positive wire attaches was being pinched and compressed between the hub and the horn ring. Had I left it as-is it would have grounded out on the hub - and the horn would be on all the time. I had to modify the tab, trimming it a bit to fit better within that space. Second issue I recall was that the hole through which the wire threads was not in the correct location, forcing the wire to be pinched; I had to modify the hub slightly to ensure it would not ground out.
I'm guessing you're running into those same packaging issues. Pull out the horn ring and ensure that nothing behind it is pinched or leaning against the hub.
Finally - and somewhat unrelated - once installed I found the steering wheel to be too far away from me and too close to the turn signal lever. That is because despite the addition of the Momo hub, the total offset with a flat Momo wheel is still less than the offset of the stock wheel. To resolve that I bought a 1" spacer from LTB Motorsport:
http://ltbautosports.com/ltbstwhbl25.htmlGA