Magnum 5.9 piston crown thickness
Is the magnum rod 6.125 ?
Factory SBM rods were 6.123 ,,,,,at least all the old stuff was .
Do you need to cut both valve notches,,,or just intake ?
Either way,,,,I believe what you did on the piston in the pic will be fine.
Looks good,,,,,I think you will be in good shape .
Tommy