Piston stop on a big block with iron heads

Am I doing it wrong? When I used a piston stop on my 318, it worked as expected, meaning there was definitely a positive stop when the piston contacted the....stop.

But with my 440, it was a lot harder to feel when the piston contacted the stop. Since I wasn't sure, I ended up going back to using a wooden dowel and marking twice, then dividing the two.

When I removed the heads, my suspicion was confirmed when I looked at the #1 piston. The piston stop simply slid along the top of the piston due to the sharper angle that the plug is at. I could tell because there was a line scratched in the piston made by the stop. I wish I took pictures when the heads were off so everyone could see, but the engine is back together.

Has anyone had this experience? What did I do wrong?