home made longer leaf spring shackles

I'm making some longer shackles for my 73 duster because my springs are basically straight and flat and my tires rub....

And this is a very cheap solution

I measured the distance from the center to center of the bolts and they are 3 1/2 inches apart 1 1/4 wide by 1/4 thick

I got four pieces cut to 8 inches long so I have room to experiment on how many inches I want it to be and thats my only set back I don't know how many inches I want to add for the lift..
I know there is some geometry in play here and need help solving how many inches these shackles would actually lift the rear compared to how many inches I actually cut them too.

So if I were too drill two holes 6 inches apart vs 3 1/2 apart that would give me 2 1/2 in lift but would it actually give me 2 1/2 of lift when put on the car

Any help would be appreciated thanks