If you can't use the original "barrel clips" (c-clips), most likely the covers are not long enough (Jim Carter sells those clips - they are a royal PITA to remove and to install - without small tears in the fabric). Can you compress/press the springs to allow you to attach the hog rings?

Advance-Design seat frames through at least 1955-1st series were made for use of barrel/c-clips.

On one of my sets of 1954 seats covers, I had to have a local upholsterer sew extra non-seen fabric around the perimeter of the covers. Then hog rings could easily be used. I paid a local upholsterer to install the covers (check a few small shops for quotes).

What types/layers of padding do you intend/plan to use?