I use an automatic center punch from the inside, in the middle of the hole in the backing plate.
This tool has a half dollar diameter handle end on it. All you do is push firmly on it and it goes "Clunk" and makes a dent. You may have to do it a few times until you see a pimple on the outside. I them use the punch to make a dimple out of the pimple amd drill the hole for the 1/4" bolts.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission