318 Magnum Dropped Valve

As noted before, take the other head off for inspection. If you wanted to, you could measure the stroke on that cylinder and the opposite cylinder with a depth mic. Amazon product ASIN B08X7LRBHY Use a dial indicator to find TDC and then measure how far the piston is below deck. Istall a timing tape with zero line upo to the zero timing mark. Summit Racing SUM-167598 Summit Racing™ Timing Tape | Summit Racing
Rotate 90 degrees and subtract below deck dimension from BDC dimension. Rotate engine over at least three times and repeat each TDC and BDC measurent each time. Ideally, the dimensions will remain close (within a few thousanths) and consistent. That will give you some indication of the condition of that piston/rod/bearing in comparison to the other one. For the time and $$$, you'd still be better off pulling the pan and that rod and pison assembly to make sure everything moves like it should and checking the rod bearing while the heads are getting checked out.