Those holes you're using as a reference are not ideal nor is the front of the frame horn. You're better off measuring from the holes in the front crossmember back as those are factory reference holes. Measure from the holes to the face of the spring hanger mount then weld the mount square to the frame rail both out and up.

Looks like your frame connectors are covering the holes so measure from the crossmember

I think if you look in a FSM it will even have those measurements for you.