Most of them mount to the front valance, 74's don't have a valance, but there have been people that have done it.
I knew they were made for the small bumpers, was hoping someone figured out how to modify it for the big ones and had pics.
if i recall correctly it will fit with the 74 bumpers bt you wont be able to see the valance but the soiler would mount to it and be visable
I had mounted a Pinto Cruiser wagon front spoiler to the front of my 1974 back in the day. It looked pretty good. Here is one on the front of Wilbur Green's car from 2006 MCCI Roundup Nationals. I am not sure where he got it. Maybe he will chime in or you can contact him.
Well now that I see one on a big bumper I'm not so sure i like it. Wow his car is almost the same color as mine. I actually sent a PM to Wilbur because I noticed he was our State Rep and he lives close by. He never responded. I hope he's ok.