Another variable is; Lining up the anvil to the frame is a critical part of the fabricating, and if it's not spot on the dies are out of wack. Been there done that.
I discovered that a digital level is a real + when fabing these critical line ups. You can zero them on the base (even if it's not level) and forget about levels and squares so much. They are within 1/10 of one degree, they're not cheap but if they prevent one mistake, they are priceless.
I have to admit my first hammer took way longer than was feasible,a good learning experience though. I could actually call it fun, except for the 6, 1'' bolt holes I had to tap.
One cool thing about building your own hammer, you get what you want......Hopefully........mb