Pull the running boards.
I replaced the seals on my '54 last month. The screws into the retainer are 3/4" clutch head sheet metal screws. Yours are probably rusted to the retainer. One is partially blocked by a running board support. Near impossible to remove them with the running boards on.
Buy new retainers and paint them with a good rustproofing paint. Run a screw into each hole in the retainer to ease the installation. Probably a good idea to sand and Ospho the cab where the retainers attach, then paint with rustproofing paint.
I replaced the old steel screws with 3/4" square drive stainless steel screws. Wipe some anti seize compound on them as you install. If a truely picky person stands on his head and glances at them above the runningboard, they almost look like clutch head screws! And they were easier to install than slotted or Phillips.
I used Fastenal SKU: 29971 "#10-12 x 3/4" Square Pan Head Sheet Metal Screw Type A, 18-8 Stainless Steel."
< CLICK >One of my running boards went back on with no problem. The other took Wayne (Wayne67vert) and me half a day to install. The front and rear fenders were too close together. Either the cab or fenders had shifted. Might as well buy new stainless hardware for the running boards, too. 'Course you can also strip and paint the boards while they're off. It never ends.